MB-12903 Move `convert_back_index_kvs_to_binary/2` to mapreduce_view

Encoding the ID b-tree key values is indexer specific, hence move it
to the indexer specific module.

Change-Id: I24d16421a3489f6127d4c72bc4c18f3382db0047
Reviewed-on: http://review.couchbase.org/44281
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>