This addresses a potential panic when a tower has one of its candidate
sessions chosen, but its only reachable address was removed by a
user-initiated RPC before the fact.
These operations are currently unused, but will be integrated into the
TowerClient at a later point as future preparation for the
WatchtowerClient RPC subserver, which will allow users to add, remove,
and list the watchtowers currntly in use.
This currently takes O(N) time as there does not exist an index of
active client sessions for each watchtower within the client's database.
This index is likely to be added in the future.
This commit adds the full bbolt-backed client database as well as a set
of unit tests to assert that it exactly implements the same behavior as
the mock ClientDB.
