Expect a bodylen of 4 for add stream packets

Change-Id: I67c3b8fee290a135d83b0af5e1e73bd8fe404f97
Reviewed-on: http://review.couchbase.org/29899
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>