ConnectionManager finally block cleans connections sequentially and can lead to delays on timeout ennforcement

[bolt12]

ConnectionManager cleans connections and cancels their connection handler thread in a sequential fashion. The logic to do so is blocking and requires locks on the connection's connVar. This can incur in delays while performing each cleanup sequentially making us fail timeout deadlines.

To mitigate this one should perform each cleanup on a different thread and wait for all cleanups to be done, since no locks are shared, in order to minimize the wait time on cleanup.