* mempool: add a safety check, write tests for mempoolIDs

and document 65536 limit in the mempool reactor spec

follow-up to https://github.com/tendermint/tendermint/pull/2778

* rename the test

* fixes after Ismail's review

Closes #1798

This is done by making every mempool tx maintain a list of peers who its received the tx from. Instead of using the 20byte peer ID, it instead uses a local map from peerID to uint16 counter, so every peer adds 2 bytes. (Word aligned to probably make it 8 bytes)

This also required resetting the callback function on every CheckTx. This likely has performance ramifications for instruction caching. The actual setting operation isn't costly with the removal of defers in this PR.

* Make the mempool not gossip txs back to peers its received it from

* Fix adversarial memleak

* Don't break interface

* Update changelog

* Forgot to add a mtx

* forgot a mutex

* Update mempool/reactor.go

Co-Authored-By: ValarDragon <ValarDragon@users.noreply.github.com>

* Update mempool/mempool.go

Co-Authored-By: ValarDragon <ValarDragon@users.noreply.github.com>

* Use unknown peer ID

Co-Authored-By: ValarDragon <ValarDragon@users.noreply.github.com>

* fix compilation

* use next wait chan logic when skipping

* Minor fixes

* Add TxInfo

* Add reverse map

* Make activeID's auto-reserve 0

* 0 -> UnknownPeerID

Co-Authored-By: ValarDragon <ValarDragon@users.noreply.github.com>

* Switch to making the normal case set a callback on the reqres object

The recheck case is still done via the global callback, and stats
are also set via global callback

* fix merge conflict

* Addres comments

* Add cache tests

* add cache tests

* minor fixes

* update metrics in reqResCb and reformat code

* goimport -w mempool/reactor.go

* mempool: update memTx senders

I had to introduce txsMap for quick mempoolTx lookups.

* change senders type from []uint16 to sync.Map

Fixes DATA RACE:

```
Read at 0x00c0013fcd3a by goroutine 183:
  github.com/tendermint/tendermint/mempool.(*MempoolReactor).broadcastTxRoutine()
      /go/src/github.com/tendermint/tendermint/mempool/reactor.go:195 +0x3c7

Previous write at 0x00c0013fcd3a by D[2019-02-27|10:10:49.058] Read PacketMsg                               switch=3 peer=35bc1e3558c182927b31987eeff3feb3d58a0fc5@127.0.0.1
:46552 conn=MConn{pipe} packet="PacketMsg{30:2B06579D0A143EB78F3D3299DE8213A51D4E11FB05ACE4D6A14F T:1}"
goroutine 190:
  github.com/tendermint/tendermint/mempool.(*Mempool).CheckTxWithInfo()
      /go/src/github.com/tendermint/tendermint/mempool/mempool.go:387 +0xdc1
  github.com/tendermint/tendermint/mempool.(*MempoolReactor).Receive()
      /go/src/github.com/tendermint/tendermint/mempool/reactor.go:134 +0xb04
  github.com/tendermint/tendermint/p2p.createMConnection.func1()
      /go/src/github.com/tendermint/tendermint/p2p/peer.go:374 +0x25b
  github.com/tendermint/tendermint/p2p/conn.(*MConnection).recvRoutine()
      /go/src/github.com/tendermint/tendermint/p2p/conn/connection.go:599 +0xcce

Goroutine 183 (running) created at:
D[2019-02-27|10:10:49.058] Send                                         switch=2 peer=1efafad5443abeea4b7a8155218e4369525d987e@127.0.0.1:46193 channel=48 conn=MConn{pipe} m
sgBytes=2B06579D0A146194480ADAE00C2836ED7125FEE65C1D9DD51049
  github.com/tendermint/tendermint/mempool.(*MempoolReactor).AddPeer()
      /go/src/github.com/tendermint/tendermint/mempool/reactor.go:105 +0x1b1
  github.com/tendermint/tendermint/p2p.(*Switch).startInitPeer()
      /go/src/github.com/tendermint/tendermint/p2p/switch.go:683 +0x13b
  github.com/tendermint/tendermint/p2p.(*Switch).addPeer()
      /go/src/github.com/tendermint/tendermint/p2p/switch.go:650 +0x585
  github.com/tendermint/tendermint/p2p.(*Switch).addPeerWithConnection()
      /go/src/github.com/tendermint/tendermint/p2p/test_util.go:145 +0x939
  github.com/tendermint/tendermint/p2p.Connect2Switches.func2()
      /go/src/github.com/tendermint/tendermint/p2p/test_util.go:109 +0x50

I[2019-02-27|10:10:49.058] Added good transaction                       validator=0 tx=43B4D1F0F03460BD262835C4AA560DB860CFBBE85BD02386D83DAC38C67B3AD7 res="&{CheckTx:gas_w
anted:1 }" height=0 total=375
Goroutine 190 (running) created at:
  github.com/tendermint/tendermint/p2p/conn.(*MConnection).OnStart()
      /go/src/github.com/tendermint/tendermint/p2p/conn/connection.go:210 +0x313
  github.com/tendermint/tendermint/libs/common.(*BaseService).Start()
      /go/src/github.com/tendermint/tendermint/libs/common/service.go:139 +0x4df
  github.com/tendermint/tendermint/p2p.(*peer).OnStart()
      /go/src/github.com/tendermint/tendermint/p2p/peer.go:179 +0x56
  github.com/tendermint/tendermint/libs/common.(*BaseService).Start()
      /go/src/github.com/tendermint/tendermint/libs/common/service.go:139 +0x4df
  github.com/tendermint/tendermint/p2p.(*peer).Start()
      <autogenerated>:1 +0x43
  github.com/tendermint/tendermint/p2p.(*Switch).startInitPeer()
```

* explain the choice of a map DS for senders

* extract ids pool/mapper to a separate struct

* fix literal copies lock value from senders: sync.Map contains sync.Mutex

* use sync.Map#LoadOrStore instead of Load

* fixes after Ismail's review

* rename resCbNormal to resCbFirstTime

This reverts commit d02c5d1e30.

This reverts commit 96a3502