Improve commit log chain marker updating

CASSANDRA-13987 addressed the commit log behavior change that was introduced with CASSANDRA-3578. After that patch was committed, aweisberg did his own review and found a bug as well as having some concerns about the configuration. He and I discussed offline, and agreed on some improvements.

Instead of requiring users to configure a deep, dark implementation detail like the commit log chained markers (via commitlog_marker_period_in_ms in the yaml), we decided it is best to eliminate thew configuration and always update the chained markers (when in periodic mode).

The bug aweisberg found was when the chained marker update is not a value that evenly divides into the periodic sync mode value, we would not sync in an expected manner. For example if the marker interval is 9 seconds, and the sync interval is 10 seconds, we would update the markers at time9, but we would then sleep for another 9 seconds, and when we wake up at time18, it is then that we flush - 8 seconds later than we should have.