Merge branch '229-transport-client' into 'master'

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.

See merge request !80