MB-9612: do some exponential backoff retries on tmperrors in xmem

Change-Id: I92dae9b1678524fe1fed7598f3b46ce5c0732a6d
Reviewed-on: http://review.couchbase.org/39373
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>