Add support for ICE consent freshness (requires libnice >= 0.1.19)

[lminiero]

As the subject says, this adds support for ICE consent freshness, which was recently (since 0.1.19) added to libnice. While we already had configurable support for using connectivity checks as keep-alives (ice_keepalive_conncheck: true in janus.jcfg), we didn't have support for consent freshness yet. The documentation for the libnice feature can be found here.

This PR adds configurable support for consent freshness too, now, via a new configuration property called ice_consent_freshness. Notice that, as explained in the documentation, enabling consent freshness automatically enables keepalive-conncheck too.

As part of this patch, we added a new ice-failed event that the Janus API can originate. If, as a consequence of enabling these features, Janus detects the PC going to a failed state after it was previously connected/ready (meaning the PC may be gone), an ice-failed event will be sent to the user to whom the handle belongs. The rationale for that is that we wait for a few seconds before closing the PC for good, which should give the user some time to possibly try and recover, e.g., via an ICE restart (in case the failed detection wasn't a truly fatal one).

While we plan to merge soon, we encourage you to test this a bit and provide feedback, most importantly to ensure enabling this feature doesn't introduce potential issues (e.g., too aggressively closing things that really shouldn't be closed, maybe because of occasional packet loss affecting connectivity checks rather than a PC that has truly gone away).

As usual, I'll backport this to 0.x too once this is deemed ready to be merged.

[lminiero]

Merging.

[lminiero]

Backported to 0.x too.