Refactor token introspection

[nbulaj]

RFC:

If the protected resource uses an OAuth 2.0 bearer token to authorize
its call to the introspection endpoint and the token used for
authorization does not contain sufficient privileges or is otherwise
invalid for this request, the authorization server responds with an
HTTP 401 code as described in Section 3 of OAuth 2.0 Bearer Token
Usage [RFC6750].

Note that a properly formed and authorized query for an inactive or
otherwise invalid token (or a token the protected resource is not
allowed to know about) is not considered an error response by this
specification. In these cases, the authorization server MUST instead
respond with an introspection response with the "active" field set to
"false" as described in Section 2.2.

Fixes #1716