MB-100 Use infinity gen_server timeout in request_throttler.

Change-Id: I1739838756a27f5a921b50f0dc25d602204deaab
Reviewed-on: http://review.couchbase.org/26561
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>