logging: Fix duplicate detection for email errors.

Our implementation of duplication detection in the Zulip email error
reporting system was buggy in two important ways:

* It did not look at the traceback, and thus considered all errors as
  the same.

* It reset the 10-minute duplicate timer every time an error happened,
  thus concealing situations where the same error was occuring more
  often than 1/10 minutes.

zerver/lib/logging_util.py

@@ -1,6 +1,8 @@
 from __future__ import absolute_import
 
+import hashlib
 import logging
+import traceback
 from datetime import datetime, timedelta
 
 # Adapted http://djangosnippets.org/snippets/2242/ by user s29 (October 25, 2010)
@@ -26,8 +28,10 @@ class _RateLimitFilter(object):
                 use_cache = False
 
             if use_cache:
-                key = self.__class__.__name__.upper()
+                tb = '\n'.join(traceback.format_exception(*record.exc_info))
+                key = self.__class__.__name__.upper() + hashlib.sha1(tb).hexdigest()
                 duplicate = cache.get(key) == 1
+                if not duplicate:
                     cache.set(key, 1, rate)
             else:
                 min_date = datetime.now() - timedelta(seconds=rate)