Move user manager types to state/api/usermanager.

Making way for UserInfo as defined in the identity spec. Moving
usermanager RPC types to the client package to avoid conflicts.
Move usermanager types, ModifyUser to apiserver.
Clean up imports.
Merge pull request #344 from cmars/id_usermanager_refactor

usermanager types refactoring

Relocating usermanager-specific API param types. We want to introduce a UserInfo type for login API purposes which conflicts with a usermanager-specific structure with the same name but different purpose.

Relocating the type to the apiserver subpackage to keep the type out of a common ever-growing state/api/params package.