Clean up startup for the pusher (#6558)

* Remove redundant python2 support code

`str.decode()` doesn't exist on python3, so presumably this code was doing
nothing

* Filter out pushers with corrupt data

When we get a row with unparsable json, drop the row, rather than returning a
row with null `data`, which will then cause an explosion later on.

* Improve logging when we can't start a pusher

Log the ID to help us understand the problem

* Make email pusher setup more robust

We know we'll have a `data` member, since that comes from the database. What we
*don't* know is if that is a dict, and if that has a `brand` member, and if
that member is a string.

tests/push/test_email.py

@@ -165,6 +165,7 @@ class EmailPusherTests(HomeserverTestCase):
         pushers = self.get_success(
             self.hs.get_datastore().get_pushers_by(dict(user_name=self.user_id))
         )
+        pushers = list(pushers)
         self.assertEqual(len(pushers), 1)
         last_stream_ordering = pushers[0]["last_stream_ordering"]
 
@@ -175,6 +176,7 @@ class EmailPusherTests(HomeserverTestCase):
         pushers = self.get_success(
             self.hs.get_datastore().get_pushers_by(dict(user_name=self.user_id))
         )
+        pushers = list(pushers)
         self.assertEqual(len(pushers), 1)
         self.assertEqual(last_stream_ordering, pushers[0]["last_stream_ordering"])
 
@@ -192,5 +194,6 @@ class EmailPusherTests(HomeserverTestCase):
         pushers = self.get_success(
             self.hs.get_datastore().get_pushers_by(dict(user_name=self.user_id))
         )
+        pushers = list(pushers)
         self.assertEqual(len(pushers), 1)
         self.assertTrue(pushers[0]["last_stream_ordering"] > last_stream_ordering)