- get_from_pool(+Address, -StreamPair) is nondet[private]
- Get an existing Keep-Alive connection to Address as StreamPair. The
caller relies on non-determinism of this predicate to try another
connection if the returned one is already closed by the peer. We
cannot rely on the non-determinism of retract/1 as that respects the
logical update view. Therefore, we must use retract/1 and commit
as retract/1 guarantees that the first retracted clause is not
already retracted.