Cleanup StartupClusterConnectivityChecker and PING Verb

This is a followup to CASSANDRA-13993. After an internal review, iamaleksey had some concerns wrt to the VERB choices as was committed; this was discussed on CASSANDRA-13993, after commit. Further, he pointed out (and provided) some optimizations for StartupClusterConnectivityChecker itself. While testing some of the proposed changes, I also discovered a small bug where the timeout for waiting for the response to the PING message was dramtically shorter than the overall timeout that is configured for StartupClusterConnectivityChecker.