Modify class ActiveRecord::QueryCache

- Modify class ActiveRecord::QueryCache to work together with
  motel-activerecord.
Fix error determines tenant on QueryCache class.
Fix error in the CheckPending middleware of ActiveRecord.

Modify the middleware to work together with motel-activerecord.
Modify lobby middleware.

- Deprecate noexistent_tenant_pafe of Motel::Manager.
- Lobby just manages the tenant switching.
- Add dependencies.
Restructuring files.
Add multithreading support.
Update the README file.
Fix error on include patches to ActiveRecord library.
Modify README file.
Update README file.
Merge branch 'feature/middlewares_and_multithreading' into develop
Update minor version to 2.1.0.
Merge branch 'release/v2.1.0' into develop