Certain operations, like waiting for a scan timestamp to be "safe" before proceeding with the scan, or waiting in the context of commit-wait in the transaction manager can easily block threads which will starve all other work.
Both these operation end up calling WaitUntilAfter() or WaitUntilAfterLocally() on the HybridClock. We should use a notification mechanism instead, i.e. NotifyMeAfter(ts, &callback, &thread_pool), NotifyMeAfterLocally(ts, &callback, &thread_pool).
Similar reversals should likely be made on the MvccManager's WaitForCleanSnapshotAtTimestamp() and WaitForCleanSnapshot().