implemented upr flow control in xdcr and addressed MB-10943

For later I'm not quite sure but it does look like at least extent of
the problem is now far smaller.

Change-Id: I6f8c92b0e8c41a365208b8416e668cbf9ddfd166
Reviewed-on: http://review.couchbase.org/37239
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>