Media transport should be kept alive during double-hold scenario

[bennylp]

In double hold situation, media will be marked as a=inactive, and currently the media will be closed. This would cause the media transport to be reinitialized when the stream is resumed.

There are two known possible failures with this behavior, with ICE and SRTP. With ICE, we unnecessarily restart ICE negotiation, which will fail if remote still has the ICE session intact. And similarly this will cause inconsistent states between local and remote SRTP endpoints if only one side restarts the session.

More over, we should keep sending keep-alive packets even when the media is marked as inactive. This would be the argument to keep ICE session alive when media is inactive.

[bennylp]

(In [3219]) Fixed #1079 (Media transport should be kept alive during double-hold scenario). Details:

• now the stream will be destroyed but the media transport will be kept alive during doublehold scenario
• small fix in SRTP to also negotiate crypto even when the media is marked as inactive, otherwise it's possible that an "optional" endpoint would create RTP/AVP offer and send it to "mandatory" endpoint, which would be rejected and cause the media port to be set to zero