action: utilize release sequence(s) for synchronization

Instead of checking only the trivial release sequence (i.e., a read-acquire
reads directly from a write-release) for establishing synchronization, make use
of the ModelChecker's more complete 'get_release_seq_head()' functionality,
then loop through all release heads and synchronize with each. This is
necessary because a read-acquire may synchronize with more than one
store-release.

Note that this step only implements support based on present knowledge. The
incomplete knowledge of the modification order, as given in mo_graph, as well
as "reading from the future" may require lazy checking.