[3.1.9 backport] Race between config poll and dbconfig update

After the fix for CBG-3817 went into main, QE picked up some tests failing.

In CBG-3817 I had removed the update to the cfgCas when updating the runtime config to the new incoming config. This was to allow an rolback of the runtime config in the event of an update error (such as collection conflict). This has exposed a race between and update to the dbconfig and the background polling for bucket db configs meaning we had situation when during update to a db config sync gateway was replacing the runtime config with the bucket config during an update. Causing mismatch between a persisted dbconfig and runtime config.