Low: crmd: quorum gain should always cause new transition

0b689055 aborted the transition on quorum loss, but quorum can also be acquired
without triggering a new transition, if corosync gives quorum without a node
joining (e.g. forced via corosync-cmapctl, or perhaps via heuristics).

This aborts the transition when quorum is gained, but only after a 5-second
delay, if the transition has not been aborted in that time. This avoids an
unnecessary abort in the vast majority of cases where an abort is already done,
and it allows some time for all nodes to connect when quorum is gained, rather
than immediately fencing remaining unseen nodes.