Major overhaul of the Client API

• Major overhaul of the Protocol state machine
  • TCP failure tolerance and reconnection
  • Proper transaction handling for REQ/REP Commands (IDENTIFY, SUB, PUB, CLS)
  • Error response handling
  • Separation of the Connection controller from NSQClient. An instance of
    NSQClient now stores connection data for a single nsqd endpoint and
    provides methods to create/access a dedicated publisher Connection and to
    create subscriber Connections.
  • Subscriber state-machine
    • Wrap and manage the ready-state of subscriber Connection
  • Handle re-subscription on re-connection of the underlying Protocol
  • Handle clean un-subscription on close of the underlying Connection
  • Use int64-native to handle message timestamps