[C10D] Implement new libuv backend for TCPStore. (#108066)
The new backend is currently under a flag 'use_libuv' in TCPStore constructor
to reduce the impact on existing users as we test it.
This is a reland of #105870 with a fix for a bad test.
Differential Revision: [D48742554](https://our.internmc.facebook.com/intern/diff/D48742554)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108066
Approved by: https://github.com/H-Huang, https://github.com/fduwjj
