disable bg manager integration w/ handoff & AAE
update riak_kv_schema_tests for bg manager being disabled by default
Expose a 2-arity from_mod function for embedded mod map
- Checks for the existence of the specified bucket type in
  riak_kv_pb_bucket:maybe_create_bucket_type/2 and
  riak_kv_pb_object:maybe_create_bucket_type/2 to properly handle
  requests for undefined bucket types (defect #875)
- Integrates feedback from @reiddraper in PR https://github.com/basho/riak_kv/pull/966
   - Adds specs to riak_kv_pb_bucket:maybe_bucket_type_exists/1,
     riak_kv_pb_bucket:maybe_create_bucket_type/2, and
     riak_kv_pb_bucket:maybe_create_bucket_type/3
   - Simplifies the case statement in riak_kv_pb_bucket:process