Saner crispin retry policy.

Summary:
Don't try again immediately, and if a maximum retry limit is hit, just
fail. (The goal is to show dead account mailsyncs on a dashboard and
have manual intervention.)

Test Plan:
I applied this patch to crispin.py and watched the carnage ensue. Seems to
behave properly:
```
@@ -247,6 +246,7 @@ class CrispinClient(object):
         self._folder_names = None
         self.log.info('Selected folder {0} with {1} messages.'.format(
             folder, select_info['EXISTS']))
+        raise socket.error("fail fail fail")
         return uidvalidity_cb(folder, select_info)

     @property
```

Reviewers: mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D164