[IMP] forwardport: processing queue reliability

The queue would get items to process one at a time, process, commit,
and go to the next. However this is an issue if one of the item fails
systematically for some reason (aka it's not just a transient
failure): the cron fails, then restarts at the exact same point, and
fails again with the same issue, leading to following items never
getting processed.

Fix by getting all the queue contents at once, processing them one by
one and "skipping" any item which fails (leaving it in place so it can
get re-processed later).

That way, even if an item causes issues, the rest of the queue gets
processed normally. The interruption was an issue following
odoo/enterprise#5670 not getting properly updated in the
backend (backend didn't get notified of the last two updates /
force-push to the PR, so it was trying to forward-port a commit which
didn't exist - and failing).

forwardport/models/forwardport.py

@@ -14,16 +14,14 @@ class Queue:
         raise NotImplementedError
 
     def _process(self):
-        while True:
-            b = self.search([], limit=1)
-            if not b:
-                return
-
-            b._process_item()
-
-            b.unlink()
-            self.env.cr.commit()
-
+        for b in self.search([]):
+            try:
+                with self.env.cr.savepoint():
+                    b._process_item()
+                b.unlink()
+                self.env.cr.commit()
+            except Exception:
+                _logger.exception("Error while processing %s, skipping", b)
 
 class BatchQueue(models.Model, Queue):
     _name = 'forwardport.batches'