cubicweb #2912807 clarification of various persistent data [validation pending]

We currently have data for session and transaction. There are accessible with:

session.data
session.transaction_data

And with get_shared_data function

With #2503918, we are getting a new intermediate concept "connection" that replace the session for database access. I sugguest the following interface:

cnx.session_data
cnx.connection_data.
cnx.transaction_data
cnx.data -> cnx.session_data for backward compat purpose.

access using get_shared_data will be dropped.

prioritynormal
typeenhancement
done in3.19.0
load0.500
load left0.000
closed by<not specified>
patch[transaction] reinstall tx.data as tx.transaction_data [applied]