Proposal: Remove addressing field

The "Addressing" field is included in each and every message. Yet, it does not offer any advantage: When a message arrives, gRPC calls the appropriate user code function, uniquely defining the request/response and its action.

An exception to this is discovery over UDP. Multiple copies of one message (e.g. a "Hello") may be send out. MDPWS R0027 proposes to use the "AppSequence" header of WS-Discovery to establish a message sequence numbering.

Proposal:

• Remove "Addressing" altogether
• Introduce "AppSequence" message according to WSD in discovery messages