BUG-1813: fix DurationStatsTracker performance

This fixes the performance issues without affecting the behaviour in the
contended case. This lowers the overhead by moving to an LongAdder,
splitting out the duration/timestamp structure and moving the division
into the read path.

A follow-up patch will rework the code organization to allow for two
different implementations, where this one is useful for highly-contented
deployments. The other implementation will use synchronized block and be
useful for things which are not likely to be contended.

Change-Id: Ic8a1d5df9d1d64460670723bac7e644733237ef7
Signed-off-by: Robert Varga <rovarga@cisco.com>