Apply proper RAII to Connection

[KitsuneRal]

This is a very bad idea generally, and the current code will leave quite a few things on the floor if setHomeserver() gets called while jobs fly over. And there's no real case for it either; so just forbid it by design. Ideally, the whole server resolution/setting could be a couple of static factories so that Connection would always be created with a sane homeserver set up; but that contradicts to the currently accustomed usage of Connection as a QML component. Maybe when we get a ConnectionManager singleton in Quotient it will be possible to shape the library API in a better way.

[KitsuneRal]

We have AccountRegistry now, that is as good a factory for Connections as we can get. Getting a Connection object from it instead of creating via new doesn't seem to contradict any code in NeoChat or Quaternion.

[KitsuneRal]

This issue is renamed to reflect that it's not just the homeserver but basically the whole login session identity that has to be preserved within a given Connection object. All pre-login things have to be treated separately, either from AccountRegistry or by a separate PreConnection kind of class with very limited number of operations available.