Greatly simplify design.

This change removes a lot of unnecessary and complicated locking (if
serializing requests is needed in the future, a goroutine will be used
instead) and also shifts the heavy lifting from frontends to btcwallet
itself to handle any notifications when they can be properly handled.
Although it's still legal to, frontends no longer need to explicitly
request account balances as these are calculated and sent as an async
notification on frontend connect, and these notifications will only
occur if btcd is currently connected.  Likewise, when btcd connects,
all frontends are immediately notified of all notifications that
require btcd information, such as the current block height for
calculating account balances.