Use a requests session to limit opened sessions
Use a requests session to limit opened sessions

We have some issues with a very high throughput of jobs and Odoo's
anonymous sessions. Every job pushed to Odoo uses a new anonymous
sessions, which creates a tremendous amount of session files.

Using a requests.session means that the session will keep the same
cookie once it receives the first response.

There is 2 problems with this implementation though:

* requests.Session is not proved to be threadsafe [0], but maybe we can
manually retrieve the session id and craft the next messages with it.
Anyway as they are all anonymous, if we lose a session at some point
there is no big deal.
* the main issue is that we don't wait for an answer so until we have a
requests responding before the timeout we won't have any session id

[0] https://github.com/requests/requests/issues/2766

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