p2p: de-dupe and shuffle public rooms; implement keydb (#1074)

- We don't want a serverKeyAPI as fetching keys doesn't need a DB.
- De-dupe rooms so we don't see them multiple times, but shuffle the
  alias we join via so we don't all flood a single server.

cmd/dendritejs/keyfetcher.go

@@ -82,3 +82,8 @@ func (f *libp2pKeyFetcher) FetchKeys(
 func (f *libp2pKeyFetcher) FetcherName() string {
 	return "libp2pKeyFetcher"
 }
+
+// no-op function for storing keys - we don't do any work to fetch them so don't bother storing.
+func (f *libp2pKeyFetcher) StoreKeys(ctx context.Context, results map[gomatrixserverlib.PublicKeyLookupRequest]gomatrixserverlib.PublicKeyLookupResult) error {
+	return nil
+}