CBD-51: Store key (MBB) as raw JSON

Previously the MBB was stored as Erlang terms, now it is stored
as raw JSON.

Change-Id: Idf94a3c1854af9439d1133713e14c947a0b2e6a4
Reviewed-on: http://review.couchbase.org/22333
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>