MB-8665: avoid computing bucket details twice for streaming case

We were already sending "stable" (i.e. without all crap and volatile
stats) output to streaming bucket details. But higher-level logic was
still to compute first stable version then "normal" (even though code
was ignoring it). We're now using special form of replying from
streaming function that makes handle_streaming facility just send back
stable output.

Change-Id: I9cfe2757c05249b51a0bb2f014d3a7fa2d43080c
Reviewed-on: http://review.couchbase.org/28959
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>