MB-5487: made xdcr stop work sooner on drop of tokens count

Concurrency throttle now detects drop of tokens count and will ask
processes holding token to release token back sooner. This is done via
return_token_please message.

xdcr vbucket replicator handles this by sending changes reader
a message to stop asap.

Change-Id: I68bea458c3799d3a15e01ed03fd4e97a7d177311
Reviewed-on: http://review.couchbase.org/33708
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>