Fix logcontext handling in verify_json_objects_for_server

preserve_context_over_fn is essentially broken, because (a) it pointlessly
drops the current logcontext before calling its wrapped function, which means
we don't get any useful logcontexts for _handle_key_deferred; (b) it wraps the
resulting deferred in a _PreservingContextDeferred, which is very dangerous
because you then can't yield on it without leaking context back into the
reactor.

Instead, let's specify that the resultant deferreds call their callbacks with
no logcontext.

synapse/crypto/keyring.py

@@ -18,7 +18,7 @@ from synapse.crypto.keyclient import fetch_server_key
 from synapse.api.errors import SynapseError, Codes
 from synapse.util import unwrapFirstError, logcontext
 from synapse.util.logcontext import (
-    preserve_context_over_fn, PreserveLoggingContext,
+    PreserveLoggingContext,
     preserve_fn
 )
 from synapse.util.metrics import Measure
@@ -83,9 +83,11 @@ class Keyring(object):
         self.key_downloads = {}
 
     def verify_json_for_server(self, server_name, json_object):
-        return self.verify_json_objects_for_server(
+        return logcontext.make_deferred_yieldable(
+            self.verify_json_objects_for_server(
                 [(server_name, json_object)]
             )[0]
+        )
 
     def verify_json_objects_for_server(self, server_and_json):
         """Bulk verifies signatures of json objects, bulk fetching keys as
@@ -95,8 +97,10 @@ class Keyring(object):
             server_and_json (list): List of pairs of (server_name, json_object)
 
         Returns:
-            list of deferreds indicating success or failure to verify each
-            json object's signature for the given server_name.
+            List<Deferred>: for each input pair, a deferred indicating success
+                or failure to verify each json object's signature for the given
+                server_name. The deferreds run their callbacks in the sentinel
+                logcontext.
         """
         verify_requests = []
 
@@ -127,9 +131,9 @@ class Keyring(object):
 
         # Pass those keys to handle_key_deferred so that the json object
         # signatures can be verified
+        handle = preserve_fn(_handle_key_deferred)
         return [
-            preserve_context_over_fn(_handle_key_deferred, rq)
-            for rq in verify_requests
+            handle(rq) for rq in verify_requests
         ]
 
     @defer.inlineCallbacks