Keep data model in sync (/buffers, /users etc.)

[clue]

The "data model" is used to represent quassel state, such as list of networks, buffers and users.

Currently, we only receive the model when connecting to quassel-core and then store these values. Whenever any of these values changes, quassel-core sends updated values to us.

We do not currently keep any of these updated values. This means that if you use the "/users" command, it may not be up-to-date.

This may depend on clue/reactphp-quassel#4