Implement naive flow control in RPC over HTTP v2 server

This allows us to send attachments greater than maximum RPC PDU size (4KiB).

In addition to this, we have fixed and added the following items:

* Fix RTS cookies parsing and construction in UUID objects as they are passed using little endian
* Pass RTS RPC commands from IN to OUT channel
    * This allows us to close the connection using a custom RTS command and pass from IN to the client RTS commands as well