Before this sortsnapshots always tried to sort source snapshots, also
when it was requested to sort target snapshots. This led to errors
when the list of target and source snapshots was not identical.
Fix it by allowing to specify an index when sorting snapshots.

This commit changes syncoid's behavior so it is always looking for
a matching snapshot and a matching bookmark. If the bookmark was created
after the snapshot it is used instead. This allows replication when the
latest snapshot replicated was deleted on the source, a common snapshot
was found but rollback on the target is not allowed. The matching bookmark
is used instead for replication.

This fixes jimsalterjrs#602