testmergeset sets up a series of replication sets, progressively
merging them together.

This was written based on "testpartition", the difference being that
in this test, tables are added to replication via explicitly creating
a set (a temporary set #999), and then merging that set back into the
main set.

This test also takes a fairly aggressive tack on usage of WAIT FOR
EVENT; when it gets into the "merge phase," it submits a SYNC followed
by waiting for that SYNC to be confirmed on all relevant nodes before
submitting a MERGE SET request to eliminate the extra set.  It does
not use any SLEEP requests; concurrency control is expected to be
controlled by WAIT FOR EVENT.