Don't cleanup the mediator task if ExpireLock message is not processed

[LefterisJP]

Problem Definition

During heavy testing with intermediate restarting we noticed that sometimes channels could get in a stuck state (inability to send transfers in one direction) and the logs had a lot of Nonce did not change sequentially, expected: X, got: X+Y entries.

After a lot of logs and debugging provided by @palango @ulope and @czepluch at the end @hackaugusto managed to find the root cause of the problem:

• For a path A-B-C
• B is the mediator, and the node that will fail
• The lock for a transfer T is sent, which expires at block N
• For some reason A doesn't respond, and the lock expires (perhaps a restart)
• The block N + confirmation is reached, and the lock is considered expired
• The node B process the block and sets the state of the lock to expired
• the bug: because the locks are expired the task is considered finalized, and cleans itself
• Because of the above, there is no task available to process the remove expired lock message, and the channel gets stuck

[LefterisJP]

Fixed by #2901