[C10D] Extract some bits of TCPStore into TCPStoreBackend. (#105163)
This moves BackgroundThread to TCPStoreBackend.hpp. This will eventually be the
interface shared between the current TCPStore backend and the new libuv one.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105163
Approved by: https://github.com/fduwjj, https://github.com/H-Huang
