Implicit media closing on BYE (hangup)

[emiago]

It is noticed that currently you are required to handle media in seperate goroutine in order to detect hangup.

This leads into lot of additional goroutines.

Still in some cases you want to handle hangup explicitly, or we need to have a way
to register OnHangup where media stack can close it self.

This will probably fix lot of issues, but it could lead also double closing, as it iss needed to close always when caller stops processing call.

[emiago]

FYI was default behavior for server but not for client