optimize order book streaming (deltas vs snapsnots)

[wlvchandler]

We need a way to send delta updates for only levels that have changed

• add bool is_snapshot to OrderBookResponse
• add uint64 sequence_number to OrderBookResponse so clients can detect missed packets
• update streaming logic:
  • the first message in a stream should be a full snapshot
  • subsequent msgs should be deltas containing only price levels where qty has changed
  • a qty of 0 in a delta ==> level should be removed