count upr snapshot marker commands for purpose of flow control

Because ep-engine counts it too.

Change-Id: I25e6360806b71cc86b04f4af1b5ddc94d32f189e
Reviewed-on: http://review.couchbase.org/37978
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>