PHP-1046: command cursor check

Figuring out why  tests/replicaset/mongocommandcursor-008.phpt  and tests/replicaset/mongocollection-aggregatecursor-003.phpt failed was fun. Turned out that sometimes the drop() migrated slow enough to the secondaries so that it would drop after the cursor was made—hence resulting in the error that this patch just added a check for.