MB-8903: Rename "stream start" to "stream request"

Align the names better with the specs and other implementations

Change-Id: Ib4fb81acbf859c6f82c8d42e5e4fad181172f544
Reviewed-on: http://review.couchbase.org/30174
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>