add token_type param
start implementing token expiration logic
stub out token refresh
first implementation of token refresh request
have TokenStore handle TokenResponse, rather than Session handling it
refactor TokenStore

We were starting to see real logic entering the mock token store
implemented for testing. That logic has been moved to the new TokenStore,
while the storage backend has been re-implemented with the session
storage interface provided by the Symfony HTTP Foundation component.

refine test for token refresh

consolidate test

remove unneeded lines