Revert switch to approximate time in Dispatcher to avoid mixing with nanoTime() in downstream timeout calculations

CASSANDRA-15241 changed Dispatcher to use the approxTime implementation of MonotonicClock rather than nanoTime(), but clock drift between the two, can potentially cause queries to time out more quickly. We should be able to revert the Dispatcher to use nanoTime() again and similarly change QueriesTable} to {{nanoTime() as well for consistency.