Helgrind thread error detector test and analysis

[ming]

Helgrind is a thread detector tools which is useful to inform us about possible data races and violation of lock ordering. Undoubtedly, it will be a useful tool to test scenarios such as registration, audio/video calls, ICE, and so on.

Ideally, we should try to achieve as fewest warnings as possible, or if that's not possible, for issues which we have thoroughly analysed and identified as safe, create a suppression file (with the help of Helgrind's gen-suppression option), which can be used for future testing.

For known issues so far, please refer to ​this wiki

Note that Helgrind test will be done primarily on Linux since its Mac support, for now, is still quite limited.