Abstract BackoffTimer.

Change-Id: I0b29895d0bc76c51de1e3a158c1899148be68df5
Reviewed-on: http://review.couchbase.org/42942
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>