Open HPI

Currently RPC header is defined as:
struct MessageHeader{ uint8_t type; uint8_t flags; uint32_t id; uint32_t len };

Its memory footprint depends on platform and also depends on compiler alignment settings.
For example on most platforms there is 2 bytes of padding between flags and id fields. And on most platforms the header size is 12 bytes.

Suggest to make the header really cross-platform by re-defining it as 12 byte uint8_t array/