Remove no longer supported functions from manual
Remove local_actor::is_running
Support ASIO-like post()/dispatch() in multiplexer
Improve output of broker-related unit tests
Remove dead code
Fix lifetime management of brokers

Solve an issue that caused brokers to live indefinitely, i.e., the destructor
of brokers was never called. Also, the runtime no longer emits
`connection_closed_msg` messages if a broker closes one of its sockets.