Add support for using keyword arguments with cached functions

Don't try & check the username if we don't have one (which we won't if it's been saved in the auth layer)

v2_alpha /register fixes for Application Services

Namely this means using registration_handler.appservice_register.

V2 Registration forced everyone (including ASes) to create a password for a
user, when ASes should be able to omit passwords. Also unbreak AS registration
in general which checked too early if the given username was claimed by an AS;
it was checked before knowing if the AS was the one doing the registration! Add
unit tests for AS reg, user reg and disabled_registration flag.

Generate local thumbnails on a thread

Pick larger than desired thumbnail for 'crop'

Improve naming

Implement read receipts.

Implement presets at room creation

Change power level semantics.

Add bulk insert events API