Add websocket handlers to the RPC server for wallet connections.

This change adds additional http listeners for websocket connections
on "/wallet".  Websockets are used to provide asynchronous messaging
between wallet daemons (i.e. btcwallet) and btcd as they allow an easy
way for btcd to provide instant notifications (instead of a wallet
polling for updates) and multiple replies to a single request.
Standard RPC commands sent over a websocket connection are handled
just like RPC, returning the same results, the only difference being
that the connection is async.  In cases where the standard RPC
commands fall short of wallet daemons requests, and to request
notifications for addresses and events, extension JSON methods are
used.

Multiple wallets can be connected to the same btcd, and replies to
websocket requests and notifications are properly routed back to the
original requesting wallet.

Due to the nature of turning a synchronous protocol asynchronous, it
is highly recommended to use the JSON id field as a type of sequence
number, so replies from btcd can be routed back to the proper handler
in a wallet daemon.