close all DtlsTransports in pc.close() (including sctp.transport) (#1565)
* close all DtlsTransports in pc.close() (including sctp.transport)
Fix for Issue https://github.com/w3c/webrtc-pc/issues/1381
Rebase of PR https://github.com/w3c/webrtc-pc/pull/1401
* Add quotation marks
* Rearrange order of closing
* Update order of steps