Remove redundant merged_keys dict

There's no point in collecting a merged dict of keys: it is sufficient to
consider just the new keys which have been fetched by the most recent
key_fetch_fns.

synapse/crypto/keyring.py

@@ -275,10 +275,6 @@ class Keyring(object):
         @defer.inlineCallbacks
         def do_iterations():
             with Measure(self.clock, "get_server_verify_keys"):
-                # dict[str, dict[str, VerifyKey]]: results so far.
-                # map server_name -> key_id -> VerifyKey
-                merged_results = {}
-
                 # dict[str, set(str)]: keys to fetch for each server
                 missing_keys = {}
                 for verify_request in verify_requests:
@@ -288,21 +284,22 @@ class Keyring(object):
 
                 for fn in key_fetch_fns:
                     results = yield fn(missing_keys.items())
-                    merged_results.update(results)
 
                     # We now need to figure out which verify requests we have keys
                     # for and which we don't
                     missing_keys = {}
                     requests_missing_keys = []
                     for verify_request in verify_requests:
-                        server_name = verify_request.server_name
-                        result_keys = merged_results[server_name]
-
                         if verify_request.deferred.called:
                             # We've already called this deferred, which probably
                             # means that we've already found a key for it.
                             continue
 
+                        server_name = verify_request.server_name
+
+                        # see if any of the keys we got this time are sufficient to
+                        # complete this VerifyKeyRequest.
+                        result_keys = results.get(server_name, {})
                         for key_id in verify_request.key_ids:
                             if key_id in result_keys:
                                 with PreserveLoggingContext():