Core: dynamic changes to max_merge_count are now picked up by index throttling

Today, index throttling won't notice any dynamic/live changes to max_merge_count.

So, I just fixed the throttle code to ask the MergeSchedulerProvider for its maxMergeCount every time a merge starts/finishes.

This means after a dynamic change, it will be the next merge that starts/finishes until the throttling notices the change.  We could also install an UpdateSettingsListener to force throttling to notice the change immediately, but that's more complex and I think this simple solution is sufficient.