MB-12048: Signature generation is indexer specific

The generation of the view group signature is no longer independent
of the type of the indexer. Now the mapreduce and the spatial index
have their own calculation.

Change-Id: I901f6044cde5848eaaf18f63234e339cf3c80c82
Reviewed-on: http://review.couchbase.org/40916
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>