Fix ThreadSafeCallback context and cleanup

[paullouisageneau]

This PR fixes two issues related to ThreadSafeCallback:

• It removes the callback context, which caused segfaults due to env becoming invalid (it should fix SegFault after IceState::Failed #245). The context seems useless as callback are not expected to be bound.
• It ensures cleanup is always called, even when the user doesn't set a closed callback, by setting dummy callbacks (fix after the close process redesign in Close without destroy #239). You can check that Jest doesn't report open handles anymore for dc1 and peer2 in p2p.test.js after running tests.

[murat-dogan]

It was on my list but I didn't have time.
Thanks @paullouisageneau

[ibsusu]

this works wonderfully!

[paullouisageneau]

@ibsusu Great, thank you for testing!