Simplify system msg handling, add addr release

Simplify system msg handling: Network Update() will return immediately
for all system message types, except for re-routed address requests or
responses. This allows additional layers (such as RF24Mesh) to transmit
and receive system payloads, and handle them directly from the
frame_buffer, without interfering with user payloads.
- Added NETWORK_ADDR_RELEASE type. Allows sleeping nodes to release
their assigned address before going down.