bundle2: read the whole bundle from stream on abort When the bundle processing abort on unknown mandatory parts, we now makes sure all the bundle content is read. This avoid leaving the communication channel in an unrecoverable state.

bundle2: add some distinction between mandatory and advisory part Mandatory part cannot be ignored when unknown. We raise a simple KeyError exception when this happen. This is very early version of this logic, see inline comment for future improvement lead.

bundle2: introduce a `parthandler` decorator Simple syntax sugar to register an handler for a new part type.