[#64570338] Fix session id bug.

Session id bug seems to appear in a multithreading environment when
another thread reuse an open connection and reset the current_session_id
useb by a previous thread.

This commit fix the bug by setting again the current_session_id at the
beginning of a transaction. Hopefully, a SQL connection shouldn't be
reused by Sequel in the middle of a transaction.
Merge pull request #229 from maxigit/64570338_session_id_not_found_

[#64570338] Fix session id bug.
Fix method name and sql request to set @current_session_id to null
Merge pull request #230 from llh1/fix_core_session_id

Fix method name and sql request to set @current_session_id to null
[fix #64570338] second attempt.
Merge pull request #231 from maxigit/64570338_session_id_not_found_

[fix #64570338] second attempt.
Initial UserSession class
Extract ForTest mock class to shared context.
Add UserSession persistence specs.
Add UserSession persistance (and fix specs)
Adding Revision::Session
Basic revision "loading" specs written.
Sequel::Revision::Persistor.bulk_load in progress

Loading works but we need a revision object
(to be able to load "deleted" object)
Add Revision::ResourceState

This allow to get some information on an object revision as well
as managing deleted object.
Specs relative to association in Revision works
Fix mix up between :internal_id and :id.
Merge pull request #232 from maxigit/63538006_desing_and_implement_access_to_revision_tables

63538006 desing and implement access to revision tables
Bumped to 3.1.0.6.4
Add 'find revision for resource' specs.
Remove persistor parameter from Persistor#create_resource_state