token_store should be an actual class/interface

[fluffy-critter]

Right now token_store is always just an itsdangerous signer, but really it should be an interface with set(value,redir) get and pop, and the default should be an implementation that is backed by the signer. get and pop should return a tuple of (value,redir). signer-backed impl uses utils.unpack_token as its implementation.