Fixes https://github.com/tendermint/tendermint/issues/1162

Databases as a service!

Can now access Databases as a remote service
via gRPC for performance and easy deployment.

The caveat is that each service is stateful in regards to
the DB i.e. each unique service uses only one unique DB
but nonetheless multiple clients can access it.

A full standalone example

```go
package main

import (
  "bytes"
  "context"
  "log"

  grpcdb "github.com/tendermint/tmlibs/grpcdb"
  protodb "github.com/tendermint/tmlibs/proto"
)

func main() {
  addr := ":8998"
  go func() {
    if err := grpcdb.BindRemoteDBServer(addr); err != nil {
      log.Fatalf("BindRemoteDBServer: %v", err)
    }
  }()

  client, err := grpcdb.NewClient(addr, false)
  if err != nil {
    log.Fatalf("Failed to create grpcDB client: %v", err)
  }

  ctx := context.Background()
  // 1. Initialize the DB
  in := &protodb.Init{
    Type: "leveldb",
    Name: "grpc-uno-test",
    Dir:  ".",
  }
  if _, err := client.Init(ctx, in); err != nil {
    log.Fatalf("Init error: %v", err)
  }

  // 2. Now it can be used!
  query1 := &protodb.Entity{Key: []byte("Project"), Value:
[]byte("Tmlibs-on-gRPC")}
  if _, err := client.SetSync(ctx, query1); err != nil {
    log.Fatalf("SetSync err: %v", err)
  }

  query2 := &protodb.Entity{Key: []byte("Project")}
  read, err := client.Get(ctx, query2)
  if err != nil {
    log.Fatalf("Get err: %v", err)
  }
  if g, w := read.Value, []byte("Tmlibs-on-gRPC"); !bytes.Equal(g, w) {
    log.Fatalf("got= (%q ==> % X)\nwant=(%q ==> % X)", g, g, w, w)
  }
}
```

See CHANGELOG

* Fix PrefixDB Iterator
* PrefixDB Iterator/ReverseIterator fixes
* Bump version 0.8.2
* Update CHANGELOG.md about DebugDB
* Keep invalid source to be closed
* Use prefixBatch instead of memBatch

Release/v0.8.1

* Use an interface for tags.

* rename TagSet to TagMap.

* add documentation to TagMap.

* added PrefixToBytes

* added test

* added comment

add SimpleProofsFromMap

Random missing methods

Add Float64() and RandFloat64().

bug fix: WriteFileAtomic

* use increment and decrement operators.

* remove unnecessary else branches.

* fix receiver names.

* remove omittable code.

* fix dot imports.

Must close file before rename it.

Bucky/merge master

* Update changelog

Refs https://github.com/tendermint/tendermint/issues/1380

* [pubsub] fix unsubscribing

by giving it the same exact query, which was used to subscribe

Refs https://github.com/tendermint/tendermint/issues/1368

* use original query to unsubscribe

Refs #1368

* modify the unit test the issue is fixed

* New Error can capture Stacktrace
* Intelligent ErrorWrap
* Review fixes

Release/v0.7.1

We can make the implementation more robust by adjusting our assumptions
and leverage explicit file modes for syncing. Additionally we going to
assume that we want to clean up and can't really recover if thos
operations (file close and removal) fail.

* utilise file mode for majority of concerns
* improve test coverage by covering more assumptions
* signature parity with ioutil.WriteFile
* always clean up

Replaces #160

Fixes #169
Fixes https://github.com/tendermint/tendermint/issues/1322

The previous code was very trusting assuming that
rational actors will use this code. However, Byzantine
actors don't care and in the case of the linked issue
negative lengths can be sent to this code unfettered
having been received from a peer.

This code is essentially just a sign change from
`==`
to
`<=`

and we've gutted out that attack by being more defensive.

update version, changelog

Simplify WriteFileAtomic

* Implement better Parallel

We can make the implementation more robust by adjusting our assumptions
and leverage explicit file modes for syncing. Additionally we going to
assume that we want to clean up and can't really recover if thos
operations (file close and removal) fail.

* utilise file mode for majority of concerns
* improve test coverage by covering more assumptions
* signature parity with ioutil.WriteFile
* always clean up

Replaces #160