The funding manager doesn't need to know the details of the underlying
storage of the opening channel state, so we move the actual store and
retrieval into the channel database.
Otherwise, we would get non-standard txn's and fail to broadcast
them when cooperatively closing a channel. This wouldn't affect
funds security as no HTLCs would be active to steal. This is just
a safety measure as we should only generate standard txn's.
In order to be consistent with other sub systems an error is now
returned from the Stop functions.
This also allows writing a generic cleanup mechanism to stop all
sub systems in case of a failure.
