* refactor: add a Mediator class to decouple tr_session and tr_port_forwarding
* refactor: add tr_port_forwarding::Mediator::privatePeerPort()
* refactor: add tr_port_forwarding::Mediator::onPortForwarded()
* chore: avoid unnecessary include of timer.h in other headers
* refactor: use a uniform timerMaker() API in mediators