Remove call to "recently" removed function in mock

when someone calls Cache.invalidate with the wrong type.

Change Cache to not use *args in its interface

Make CacheDescriptor cache deferreds rather than the deferreds' values

Merge branch 'erikj/cache_deferreds' of github.com:matrix-org/synapse into erikj/cache_varargs_interface

Merge branch 'erikj/cache_deferreds' of github.com:matrix-org/synapse into erikj/cache_varargs_interface

Three small improvements to help debian package (SYN-420)

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.

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