MB-11857: Add additional log message

The DCP client was waiting for a message, but it was not clear as it
wasn't logged.

Change-Id: I4dce881eaed4e9f038f774cbf695f476eb681677
Reviewed-on: http://review.couchbase.org/40107
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>