Explicitly provide a DB context when executing a DB transaction

[Rodolfo Alonso]

Currently, when Neutron executes DB transaction, if there is no active session (or session at all), ``neutron_lib.context.Context`` creates a default writer session [1].

That's a safeguard for the code written in Neutron but an inefficient way to access to the DB. Reader context allows, for example, the ASYNC_READER mode:
"""
This state indicates that the transaction is a read-only and is
safe to use on an asynchronously updated slave database.
"""

That is supposed to be less blocking than a writer context.

When the ``neutron_lib.context.Context.session`` property is removed, if a transaction is not correctly executed inside a context (reader or writer), the transaction fails.

This bug is just a tracker for all the patches needed to decorate all the DB transactions.

[1]https://github.com/openstack/neutron-lib/blob/c2879ec1739d1425bcc762d4b6584ee241f995ca/neutron_lib/context.py#L157-L161
[2]https://review.opendev.org/c/openstack/neutron-lib/+/791978/4/neutron_lib/context.py#b142