Add mock indexer test infrastructure for testing index scanner

Implemented a mock indexer that can be used to create indexes
using mock datastructures for index instance map, index partition map,
partitions, slices and snapshots.

This can be used to write unit tests for scan coordinator.

Change-Id: I316234605509d57b4cbfdf8f4bfc947d5cb7a1be
Reviewed-on: http://review.couchbase.org/43034
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>