Mark device list as stale, if we don't have the requesting device (#2728)

This hopefully makes E2EE chats a little bit more reliable by re-syncing
devices if we don't have the `requesting_device_id` in our database. (As
seen in
[Synapse](c52abc1cfd/synapse/handlers/devicemessage.py (L157-L201)))

keyserver/inthttp/server.go

@@ -16,6 +16,7 @@ package inthttp
 
 import (
 	"github.com/gorilla/mux"
+
 	"github.com/matrix-org/dendrite/internal/httputil"
 	"github.com/matrix-org/dendrite/keyserver/api"
 )
@@ -70,4 +71,9 @@ func AddRoutes(internalAPIMux *mux.Router, s api.KeyInternalAPI) {
 		QuerySignaturesPath,
 		httputil.MakeInternalRPCAPI("KeyserverQuerySignatures", s.QuerySignatures),
 	)
+
+	internalAPIMux.Handle(
+		PerformMarkAsStalePath,
+		httputil.MakeInternalRPCAPI("KeyserverMarkAsStale", s.PerformMarkAsStaleIfNeeded),
+	)
 }