The OS X kernel subscribers have not been starting because they expect the
publisher thread to run before they begin configuration. Due to some recent
refactors the publisher thread creation now occurs after configuration.
The subscriber logic to check for a valid kernel connection is still valid.
This commit has two additional side-effects:
- The RocksDB plugin is modified to use 3 background merge threads.
- The OS X kernel publisher syncing thread is now non-blocking.
