Critical Description
When replication is resumed, XDCR launches an updater to start a replication instance. XDCR has a separate go routine that periodically scans existing replication specs and start replications by launching its own updaters (in case that the "regular" updater fails for whatever reason). An issue in the code makes it possible for both updater to successfully start replication instances, which can cause both performance issues and functional issues (since both instances access the same expvar space for stats collection).
