[bp] MB-9467 Raise explicit error if list of emitted keys exceeds limit

If more than 64k keys are emitted for a single document, raise an
explicit error informing about this fact, instead of having a badmatch
later when updating the key list for a document.

Change-Id: I61f5aa040ce026777aac52217a696f220da3d3ae
Reviewed-on: http://review.couchbase.org/30054
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-on: http://review.couchbase.org/30158