MB-8708 Allow for multiple reduces in view btree reductions

A single view btree's reduction can encode multiple values
coming from different reduce functions (views).

Change-Id: Ief273ebbbb1e29b873fec48a8c6adcdf44f5beda
Reviewed-on: http://review.couchbase.org/27683
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>