Added notifyallnewtxs custom websocket command

Changed mempool.MaybeAcceptTransaction to accept an additional parameter
to differentiate betwee new transactions and those added from
disconnected blocks.

Added new fields to requestContexts to indicate which clients want to
receive all new transaction notifications.

Added NotifyForNewTx to rpcServer to deliver approriate transaction
notification.

Depends on conformal/btcws#5.