We'd never decrement the number of pending backups upon a watchtower
accepting one, making it confusing for users to determine whether their
backups have actually been accepted. Along the way, we also rename
NumTasksReceived to NumTasksPending to better reflect its purpose.
In this commit, we extend the wtclient.Client interface with the
following methods:
* AddTower
* RemoveTower
* RegisteredTowers
* LookupTower
* Stats
Care has been taken to ensure that any in-memory state updates are
_only_ performed after a successful database update.
These methods are currently unused, but they serve as a dependency for
the upcoming WatchtowerClient RPC subserver.
