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] psf/requests#2766

queue_job/jobrunner/runner.py

@@ -176,6 +176,9 @@ def _odoo_now():
     return _datetime_to_epoch(dt)
 
 
+session = requests.Session()
+
+
 def _async_http_get(scheme, host, port, user, password, db_name, job_uuid):
     # Method to set failed job (due to timeout, etc) as pending,
     # to avoid keeping it as enqueued.
@@ -202,7 +205,7 @@ def urlopen():
                 auth = (user, password)
             # we are not interested in the result, so we set a short timeout
             # but not too short so we trap and log hard configuration errors
-            response = requests.get(url, timeout=1, auth=auth)
+            response = session.get(url, timeout=1, auth=auth)
 
             # raise_for_status will result in either nothing, a Client Error
             # for HTTP Response codes between 400 and 500 or a Server Error