Fix TCPStoreTest and improve tcputils::connect() (#30354)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30354
TCPStoreTest would timeout since the TCPStore constructor for the
server would block the main thread waiting for workers. The workers themselves
were spawned later on once the server store is created. As a result, this test
would always timeout.
To fix the test, I moved the server store to a thread so that the workers can
register with the server in parallel.
In addition to this made a few improvements to tcputils::connect. When
tcputils::connect() encountered an exception, it always looked at `errno` for
the error code. In some cases `errno` could be overwritten and the real error
code would be stored in `std::system_error`. As a result, I've modified the
code to look at the error code in `std::system_error` if we catch an exception
of that type.
ghstack-source-id: 94758939
Test Plan: waitforbuildbot
Differential Revision: D18668454
fbshipit-source-id: d5a3c57b066b094bfecda9a79d9d31bfa32e17f0
