Add an optional 'sessionId' parameter to ProtocolMessage

[EricCornelson]

A first step in enabling a "flat session" mode of communication in the protocol (see #79 for more details).

[weinand]

Multiplexing sessions and the concept of a sessionId is not part of the DAP spec.
Having a sessionId as an optional property on the ProtocolMessage is not needed and violates the concept of "Separation of concerns".

If some other entity wants to multiplex sessions over a single communication channel it can do so by wrapping a ProtocolMessage in another structure that has a sessionId property.