MB-11435: implemented xdcr anticipatory delay

Current thinking is that xdcr consuming mutations from memory rather
than from disk makes xdcr batches much smaller than it was, causing
higher cpu overhead per Mutation/sec.

Proposed treatment is to introduce delay between seeing activity in
vbucket and waking up replicator.

With this patch I was clearly able to see how under constant load of
333 mutations/sec I was able to lower number of active replicators
from 16 down to 1-2 with delays in range of few seconds.

Delay is controlled by new (private and not to be documented yet)
internal setting named: xdcrAnticipatoryDelay. Which is number of
milliseconds for delay. Default value is 0.

Change-Id: I66fe76e6b45d3bcb2cf2c6eadd653add9ccef6cd
Reviewed-on: http://review.couchbase.org/38608
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>