History: in-memory blockhash index

[ScottyPoi]

Refactors the BlockIndex into an in-memory attribute of the HistoryNetwork class.

1. Removes async blockIndex getter/setter from BaseNetwork
2. Adds this.blockHashIndex to HistoryNetwork
3. Moves hashToNumber and numberToHash helpers to HistoryNetwork
4. Makes helper functions syncronous
5. In history.indexBlockHash(...), updated index is also stored in DB
6. Stored block index is retrieved from DB during client.start() to set history.blockHashIndex

history.blockHashIndex is type Map<string,string>

Map index entries stored in both directions (num > hash / hash > num)

blockNumber is 0x prefixed hex
blockHash is 0x prefixed hex

e.g.

 Map {
   "0x1" => "0x88e96d4537bea4d9c05d12549907b32561d3bf31f45aae734cdc119f13406cb6",
   "0x88e96d4537bea4d9c05d12549907b32561d3bf31f45aae734cdc119f13406cb6" => "0x1",
 }

[acolytec3]

LGTM