Transport properties client. #229
This branch moves support for exchanging transport properties from the sync layer to the client layer.
Local properties are stored in an unshared group. When a contact is added, the latest local properties are copied into a group shared with the contact. Whenever the local properties are updated, they're copied into the groups shared with all contacts.
The transport update messages include a unique device ID that prevents collisions between local and remote messages, and will be used in future for multi-device support.