Turn _start_key_lookups into an inlineCallbacks function

... which means that logcontexts can be correctly preserved for the stuff it
does.

get_server_verify_keys is now called with the logcontext, so needs to
preserve_fn when it fires off its nested inlineCallbacks function.

Also renames get_server_verify_keys to reflect the fact it's meant to be
private.

synapse/crypto/keyring.py

@@ -123,7 +123,7 @@ class Keyring(object):
 
             verify_requests.append(verify_request)
 
-        self._start_key_lookups(verify_requests)
+        preserve_fn(self._start_key_lookups)(verify_requests)
 
         # Pass those keys to handle_key_deferred so that the json object
         # signatures can be verified
@@ -132,6 +132,7 @@ class Keyring(object):
             for rq in verify_requests
         ]
 
+    @defer.inlineCallbacks
     def _start_key_lookups(self, verify_requests):
         """Sets off the key fetches for each verify request
 
@@ -151,19 +152,15 @@ class Keyring(object):
             for rq in verify_requests
         }
 
-        with PreserveLoggingContext():
-
         # We want to wait for any previous lookups to complete before
         # proceeding.
-            wait_on_deferred = self.wait_for_previous_lookups(
+        yield self.wait_for_previous_lookups(
             [rq.server_name for rq in verify_requests],
             server_to_deferred,
         )
 
         # Actually start fetching keys.
-            wait_on_deferred.addBoth(
-                lambda _: self.get_server_verify_keys(verify_requests)
-            )
+        self._get_server_verify_keys(verify_requests)
 
         # When we've finished fetching all the keys for a given server_name,
         # resolve the deferred passed to `wait_for_previous_lookups` so that
@@ -227,7 +224,7 @@ class Keyring(object):
             self.key_downloads[server_name] = deferred
             deferred.addBoth(rm, server_name)
 
-    def get_server_verify_keys(self, verify_requests):
+    def _get_server_verify_keys(self, verify_requests):
         """Tries to find at least one key for each verify request
 
         For each verify_request, verify_request.deferred is called back with
@@ -312,7 +309,7 @@ class Keyring(object):
                     if not verify_request.deferred.called:
                         verify_request.deferred.errback(err)
 
-        do_iterations().addErrback(on_err)
+        preserve_fn(do_iterations)().addErrback(on_err)
 
     @defer.inlineCallbacks
     def get_keys_from_store(self, server_name_and_key_ids):