[services/wallet] Several changes in API after feedback
- Timestamp from block header stored in blocks table and added to each transfers
- From field is computed from signature to offload this computation from client side
- `history` event is added back, so that client can ignore historical blocks when watching
only for new blocks
- block number and timestamp are marshalled in hex. consistent with ethereum data structures
Pedro Pombeiro
Dmitry Shulyak