FileChangeWatcher improvements

HBASE-27347 ported FileChangeWatcher from ZK so that we can reload certificates when the keystore/truststore files change. I'd like to make a few improvements:

1. Add more context to the thread name, so that we can disambiguate when multiple are launched for different cluster connections or paths
2. We currently launch a separate FileChangeWatcher for each of keystorePath and truststorePath, but in many cases they may be the same path. The runnable which gets called is identical for these, so this just results in extra work for no reason. We should check the configured paths and only launch one FileChangeWatcher if the two paths are identical.