Fix replication checkpoint frequency

The patch to track and checkpoint last_seq values seen in the _changes
feed resulted in high frequency updates to the checkpoint document. This
is because the comparison made for "needs a checkpoint" was including
the replicator's incrementing counter which the changes reader was
bumping each time it saw last_seq even if it was the same last_seq as
the previous time.

This just compares the new last_seq value to the old last_seq value and
if they're equal then skips notifying the replication manager so that no
superfluous checkpoint is made.