Fix logcontxt leak in keyclient (#2465)

preserve_context_over_function doesn't do what you want it to do.

synapse/crypto/keyclient.py

@@ -13,14 +13,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
+from synapse.util import logcontext
 from twisted.web.http import HTTPClient
 from twisted.internet.protocol import Factory
 from twisted.internet import defer, reactor
 from synapse.http.endpoint import matrix_federation_endpoint
-from synapse.util.logcontext import (
-    preserve_context_over_fn, preserve_context_over_deferred
-)
 import simplejson as json
 import logging
 
@@ -43,14 +40,10 @@ def fetch_server_key(server_name, ssl_context_factory, path=KEY_API_V1):
 
     for i in range(5):
         try:
-            protocol = yield preserve_context_over_fn(
-                endpoint.connect, factory
-            )
-            server_response, server_certificate = yield preserve_context_over_deferred(
-                protocol.remote_key
-            )
+            with logcontext.PreserveLoggingContext():
+                protocol = yield endpoint.connect(factory)
+                server_response, server_certificate = yield protocol.remote_key
                 defer.returnValue((server_response, server_certificate))
-            return
         except SynapseKeyClientError as e:
             logger.exception("Error getting key for %r" % (server_name,))
             if e.status.startswith("4"):