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 https://github.com/murat-dogan/node-datachannel/issues/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 https://github.com/murat-dogan/node-datachannel/pull/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!