Refactor direct connect delivery pipeline and message store.

1) Make message store contents more memory efficient.

2) Make notification pipeline simpler and more memory efficient.

3) Don't b64 encode websocket message bodies.

// FREEBIE

protobuf/OutgoingMessageSignal.proto

@@ -36,4 +36,4 @@ message OutgoingMessageSignal {
 //  repeated string destinations = 4;
   optional uint64 timestamp = 5;
   optional bytes message = 6;
-}
+}