multibackend part of .schema belongs into riak_kv

Hi I think the multibackend part of the .schema file does not belong into the eleveldb.schema file. It's specific to riak_kv and not to this library so I think it would belong there.

This can of cause be argued given eleveldb was developed for riak_kv but still for library users it would make more sense to keep those separated.

An alternative would be having two .schema files one eleveldb.schema and one eleveldb_multi.schema