conncheck: fix long time to connect if conncheck list gets filled late

conncheck: fix long time to connect if conncheck list gets filled late

When a stream's conncheck list is empty during
priv_conn_check_unfreeze_related() and the check pairs get added only
afterwards, the whole list may remain in frozen state for tens of
seconds until the lists of all other streams are completed.

If a connection check list missed the opportunity to be unfrozen
according to the process in RFC 5245 Section 7.1.3.2.3 because it was
empty at the time, unfreeze it immediately once some check pair gets
added.