[session] Drop long deprecated [get/set]_shared_data methods they are deprecated since 3.19 and access to the .data attribute itself deprecated in 3.25, so it's time to drop them.

[session] Drop the user session synchronization machinery which should not be necessary anymore since groups and properties are fetched for each request (cache lives in transaction_data).

[session] Avoid deprecation warning on access to Connection.data this attribute has been deprecated during the session related rework, remove warning by explicitly storing data in .transaction_data since session data is not anymore reachable through Connection object. This will allows simplification of session synchronization hooks in a later cset. Also since we do not rely anymore on session data here, we may at some point rewrite those properties to rely on the ORM instead of such custom caching.