MEDIUM: sessions: Don't be responsible for connections anymore.

MEDIUM: sessions: Don't be responsible for connections anymore.

Make it so sessions are not responsible for connection anymore, except for
connections that are private, and thus can't be shared, otherwise, as soon
as a request is done, the session will just add the connection to the
orphan connections pool.
This will break http-reuse safe, but it is expected to be fixed later.