Updates #30

Adds a backend for BadgerDB as requested at
#30 (comment)
and tests for it along.

BadgerDB has a few problems:
a) It doesn't have a BatchDelete, our fallback is thus Delete
because if do DeleteAsync, by the time batch.Write is invoked,
we can't be sure that the deletions have ran
b) Retrieving values from iterators requires a secondary allocation
equal to the length of the current key.
See https://godoc.org/github.com/dgraph-io/badger#KV.NewIterator
c) It seems to have arbitrary options for the size of the
log file varying between 10MB to 2GB. Currently with our
DB API, we can't pass in the required file size unless we
invoke:
```go
db.NewBadgerDBWithOptions(&db.Options{
  ValueLogFileSize: desiredFileSize,
})
```

Anyways, I've arbitrarily set the default value log file size to 1GB
given that Tendermint does a whole lot of heavy lifting with
keyvalue stores <-- cite my sources

TODO:
- IteratorPrefix does not count in prefix
- badgerDBIterator#Error should return err from Key() or Value()

Fixed the code + tests to conform to *vet warnings
and also add a getter and setter for badgerDBIterator's
lastErr.

Thanks to @melekes for the suggestions.

Uncovered dgraph-io/badger#308
which perhaps will require us to use the latest BadgerDB
since they no longer use v0.8(which we started with)
plus until that issue is resolved, our benchmarks are invalid
or spuriously error out.