Fixes https://github.com/tendermint/tendermint/issues/751.

Adds jitter to our exponential backoff to mitigate a self DDOS
vector. The jitter is a randomly picked percentage of a second
whose purpose is to ensure that each exponential backoff retry
occurs within (1<<attempts) == 2**attempts, but with the delay
each client will have a random buffer time before it tries to
reconnect instead of all at once reconnections that might even
bring back the previous conditions that might have caused the
dial to the WSServer to have failed e.g
* Network outage
* File descriptor exhaustion
* False positives from firewalls
etc

This cleans up some of the code in the state package

https://github.com/tendermint/tendermint/pull/724#issuecomment-335362927

https://github.com/tendermint/tendermint/pull/724#issuecomment-335316484

so they can be used in rpc/core/types/responses.go.

```
So, it seems like we could use the actual structs here, but we don't want to have to import consensus to get them, as then clients are importing too much crap. So probably we should move some types from consensus into consensus/types so we can import.

Will these raw messages be identical to:

type ResultDumpConsensusState struct {
  RoundState cstypes.RoundState
  PeerRoundStates map[string]cstypes.PeerRoundState
}
```
https://github.com/tendermint/tendermint/pull/724#discussion_r143598193

Before:

```
{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "round_state": "RoundState{\n  H:10 R:0 S:RoundStepNewHeight\n  StartTime:     2017-10-09 13:07:24.841134374 +0400 +04\n  CommitTime:    2017-10-09 13:07:23.841134374 +0400 +04\n  Validators:    ValidatorSet{\n      Proposer: Validator{EF243CC0E9B88D0161D24D733BDE9003518CEA27 {PubKeyEd25519{2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202}} VP:10 A:0}\n      Validators:\n        Validator{EF243CC0E9B88D0161D24D733BDE9003518CEA27 {PubKeyEd25519{2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202}} VP:10 A:0}\n    }\n  Proposal:      \u003cnil\u003e\n  ProposalBlock: nil-PartSet nil-Block\n  LockedRound:   0\n  LockedBlock:   nil-PartSet nil-Block\n  Votes:         HeightVoteSet{H:10 R:0~0\n      VoteSet{H:10 R:0 T:1 +2/3:\u003cnil\u003e BA{1:_} map[]}\n      VoteSet{H:10 R:0 T:2 +2/3:\u003cnil\u003e BA{1:_} map[]}\n    }\n  LastCommit: VoteSet{H:9 R:0 T:2 +2/3:947F67A7B85439AF2CD5DFED376C51AC7BD67AEE:1:365E9983E466 BA{1:X} map[]}\n  LastValidators:    ValidatorSet{\n      Proposer: Validator{EF243CC0E9B88D0161D24D733BDE9003518CEA27 {PubKeyEd25519{2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202}} VP:10 A:0}\n      Validators:\n        Validator{EF243CC0E9B88D0161D24D733BDE9003518CEA27 {PubKeyEd25519{2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202}} VP:10 A:0}\n    }\n}",
    "peer_round_states": []
  }
}
```

After:

```
{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "round_state": {
      "Height": 1691,
      "Round": 0,
      "Step": 1,
      "StartTime": "2017-10-09T14:08:09.129491764+04:00",
      "CommitTime": "2017-10-09T14:08:08.129491764+04:00",
      "Validators": {
        "validators": [
          {
            "address": "EF243CC0E9B88D0161D24D733BDE9003518CEA27",
            "pub_key": {
              "type": "ed25519",
              "data": "2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202"
            },
            "voting_power": 10,
            "accum": 0
          }
        ],
        "proposer": {
          "address": "EF243CC0E9B88D0161D24D733BDE9003518CEA27",
          "pub_key": {
            "type": "ed25519",
            "data": "2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202"
          },
          "voting_power": 10,
          "accum": 0
        }
      },
      "Proposal": null,
      "ProposalBlock": null,
      "ProposalBlockParts": null,
      "LockedRound": 0,
      "LockedBlock": null,
      "LockedBlockParts": null,
      "Votes": {},
      "CommitRound": -1,
      "LastCommit": {},
      "LastValidators": {
        "validators": [
          {
            "address": "EF243CC0E9B88D0161D24D733BDE9003518CEA27",
            "pub_key": {
              "type": "ed25519",
              "data": "2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202"
            },
            "voting_power": 10,
            "accum": 0
          }
        ],
        "proposer": {
          "address": "EF243CC0E9B88D0161D24D733BDE9003518CEA27",
          "pub_key": {
            "type": "ed25519",
            "data": "2E0B9301334FCDAB193D514022F81BA09BBEC028685C96602BE9DD0BD4F9E202"
          },
          "voting_power": 10,
          "accum": 0
        }
      }
    },
    "peer_round_states": {
      "75EC8F15D244A421202F9725CD4DE509EE50303670310CF7530EF25E2B7C524B": {
        "Height": 1691,
        "Round": 0,
        "Step": 1,
        "StartTime": "2017-10-09T14:08:08.563251997+04:00",
        "Proposal": false,
        "ProposalBlockPartsHeader": {
          "total": 0,
          "hash": ""
        },
        "ProposalBlockParts": null,
        "ProposalPOLRound": -1,
        "ProposalPOL": null,
        "Prevotes": null,
        "Precommits": null,
        "LastCommitRound": 0,
        "LastCommit": null,
        "CatchupCommitRound": -1,
        "CatchupCommit": null
      }
    }
  }
}
```

I don't want to lose any documentation. Correct me if I am wrong, but we
don't have this docs anywhere else.

callback gives more power to the publisher. plus it is optional
comparing to a channel, which will block the whole client if you won't
read from it.

* rpc: subscribe on reconnection

* rpc: fix unit tests

otherwise, we're not testing ping/pongs.
see https://github.com/tendermint/tendermint/pull/687#issuecomment-332494735

Users can now just pass an object that implements the Signer interface.

if somebody tries to access WS method in non-ws context

from the spec:
This member is REQUIRED.
It MUST be the same as the value of the id member in the Request Object.
If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be Null.

```
data
A Primitive or Structured value that contains additional information about the error.
This may be omitted.
The value of this member is defined by the Server (e.g. detailed error information, nested errors etc.).
```

for all of them except unsafe

server:
- always has read & write timeouts
- ping handler never blocks the reader (see A)
- sends regular pings to check up on a client

A:
at some point server write buffer can become full, so in order not to
block reads from a client (see
https://github.com/gorilla/websocket/issues/97), server may skip some
pongs. As a result, client may disconnect. But you either have to do
that or block the reader. There is no third way.

client:
- optional read & write timeouts
- optional ping/pong to measure latency