[ale] Limit the amount of data that's being processed by worker.

And drop incoming messages on the floor if it's exceeded.

Change-Id: I33d744b3cf112244f756a8c6a01b565ad273b474
Reviewed-on: http://review.couchbase.org/37806
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>