MB-100: use cast for timeout_diag_logger

Because we don't need to wait until this guy is done and because we
already have logic of doing nothing if it's busy.

Change-Id: I793a5014ecaaa86cc3a39619fd30d03ab3b82f46
Reviewed-on: http://review.couchbase.org/27582
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>