[bp] MB-100 Use infinity gen_server timeout in request_throttler.

Change-Id: I9815987273cbff6e557dbd29e98f8d407b1f5d43
Original-Reviewed-on: http://review.couchbase.org/26561
Original-Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Original-Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-on: http://review.couchbase.org/26563
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>