[FIX] service: clear environments between transaction retries

Initial bug report, full of red herrings:
receive two support requests by email while on Odoo.sh, the first creates a
ticket, while the second replies to the server with the error:
550 Requested action not taken: mailbox unavailable

When a request is done, the service check function tries each request multiple
times when getting Operational errors.
When two email are received within a 'short' amount of time (e.g .1 second)
the two requests are initiated with a new cursor.
When both mails generate the creation of a new record of the same model,
the second cursor is going to fail because of an:
ERROR: could not serialize access due to concurrent update
However, the record we used in the computations needed in the creation of the
second record have been added to the environment.
When that second creation is retried, it will operate on the same records,
and find them in the todo, with the environment that was obtained for the first
trial. Since the cursor is closed, this crashes.

To explain the bug report, Odoo.sh transfers the mail through an xmlrpc
to process the mail.
Because of this crash, the server interprets the lack of well-formed response
by a generic 'mailbox unavailable'.

opw 2060476

closes odoo/odoo#37085

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>