MB-8199 Prevent request_throttler from crash if it restarts.
MB-8199 Prevent request_throttler from crash if it restarts.

Some of our REST endpoints restart almost all the supervision
tree. Previously note_request_done of such request would be handled
already by restarted process. This would cause it to crash and would
fail the request. To prevent this note_request returns throttler pid
as part of response. Then note_request_done uses this pid. So that new
instance of request throttler is unaffected. Additionally,
note_request_done now uses gen_server:cast instead of gen_server:call
to prevent request handlers from random request_throttler failures.

Change-Id: I9890d586fdfc25364de9153ae54925cb639a0c82
Reviewed-on: http://review.couchbase.org/26356
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

membase/ns_server,daverigby/ns_server,nimishzynga/ns_server,vzasade/ns_server,membase/ns_server,pavel-blagodov/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,mschoch/ns_server,vzasade/ns_server,membase/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,vzasade/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,membase/ns_server,vzasade/ns_server,nimishzynga/ns_server