Protocol: meeting update needs rework

[pierluca]

The meeting update property is severely lacking and should probably mirror the LAO update logic.

LAO update logic

A reminder of the LAO update logic:

1. The organizer sends a lao/update_properties message, ignored by everyone except witnesses
2. The witnesses send signatures, signing the lao/update_properties message
3. When the organizer has collected a quorum of signatures, it sends the lao/state pointing to the original lao/update_properties message and containing the collected signatures.

This ensures that clients only need to process the lao/update_properties and lao/state message, once all the witnesses have already signed off on it.

The meeting update logic

Right now, there are only two messages message/create and message/state. meeting/state expects the same signature-collecting-logic as lao/update_properties, but it's missing a corresponding meeting/update_properties. Furthermore, meeting/state suffers from the same problems as #255 and #256.

meeting/update_properties should be created, and meeting/state should be updated.