MB-12903 Move `encode/decode_key_docid` to `mapreduce_view` module

The encoding of the key is module specific. In order to be able to call
the encoding/decoding from the updater easily it makes sense to have it
in the indexer specific module.

Change-Id: Ifcc679b0c4c0553c4d98b106a2b3ebc6f5ccb746
Reviewed-on: http://review.couchbase.org/44235
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>