feat(bulk-load): add meta_bulk_load_service and some structures

[hycdong]

This pull request includes:

• Structure definition:
  • is_bulk_loading flag for app_info
  • bulk load related structure on zk: app_bulk_load_info and partition_bulk_load_info
  • bulk load related structure on remote file provider: bulk_load_info and bulk_load_metadata
• Init and start bulk load service while starting meta service, create bulk_load dir on zk
• Add other basic varieties, structures and function deceleration.

[levy5307]

I think you can change the type of bulk_load_constant from class to struct, because the member variables are all public.

[hycdong]

I used class here is just like replica_envs class, I would like to organize const varieties who used in a same function in a namespace.