ICE/PJNATH - use different Ta timer for TCP candidates
The current Ta timer (interval between two connectivity checks, see RFC-5245 for details) is set to 20ms and used for UDP and TCP candidates alike. RFC-5245 (section recommends using a less a higher value for non-real-time application (TCP/TLS connections in our case). Furthermore, newer ICE spec (RFC-8445), recommends a min value of 50ms for real-time applications. The advantage of using a low Ta value (20ms) is to start the connectivity checks, but the drawback is a abrupt increase in bandwidth (as described in RFC-5245 and RFC-8445). The peak in bandwidth usage may be significant if the candidate list is large. The goal is to find a better trade-off between connection speed and bandwidth usage.