[3/n] [c10d] Revise the implementation of TCPStore (#58330)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58330
This PR is part of a stack that addresses the GitHub issue #41614; it introduces a major refactoring of the `TCPStore` class in preparation of the multi-tenancy feature.
- All TCP sockets are wrapped with a new `TCPSocket` RAII type.
- `BackgroundThread` and daemon types are moved from header to cpp file.
- Server, client, and callback sockets are refactored into their own internal types `TCPServer`, `TCPClient` and `TCPCallbackClient`.
- Calls to `tcputil::send*` and `tcputil::recv*` are wrapped in `TCPClient` for easier readability and maintenance purposes.
- Two `TODO` statements are put to reference future improvements. Based on feedback, I will either create separate GitHub issues for them or address them as part of this stack.
ghstack-source-id: 130676392
Test Plan: Run the existing tests since there are no user-facing behavioral changes.
Reviewed By: H-Huang
Differential Revision: D28448981
fbshipit-source-id: 415b21e74b3cd51d673c1d5c349c6a2cb21dd667
