Add Session#with_subsession.
cache.

Object don't have to be 'to save' anymore to be considered as managed.
Merge pull request #223 from maxigit/61029592_use_global_session_within_lims-api

Add Session#with_subsession.
Setup the persistor modules in persistor.rb
Add comments
Use the persistor module map from Persistence::Persistor
Remove the logic related to persistor modules from session
Rebase