Force SpiderMonkey to garbage collect in ReplSetTest.checkOplogs

Certain tests, like the backup_restore_*.js tests, set a large oplog size. If the oplog becomes large (especially if there is a concentration of particularly large entries), SpiderMonkey can cause an OOM when checking the oplogs. This should be able to be fixed by manually having SpiderMonkey garbage collect (at least if the oplog is large), which is what the itcount function currently does to avoid this issue.