Check if the send buffer is full or not for an DCP mutation.

Change-Id: I8a1a9557f83076a7da90659b25757a2972f1d365
Reviewed-on: http://review.couchbase.org/40394
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>