Overhaul connection management

[TobiasFella]

As described in #781

Roughly:

• Turn Connection into a type that only exists in an initiated state (from the API user's perspective, at least)
• To do this, pull connection setup, etc. out into a new PendingConnection type
• Add easy-to-use account login and restoration functions to AccountRegistry
• Make connections do the expected things (cache, sync loop, etc.) by default

Still an early draft. Rough list of missing things:

• SSO login
• Registration
• Error handling during login
• Implementation of mockConnection
• Putting back some things that were hacked out
• Make sure that tests still work
• Homeserver class