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ARROW-15181: [C++][FlightRPC] Fix race between signal handler and shutdown

The Flight server spawns a background thread that reads from a pipe, then shuts down the server. (The pipe is written to by a signal handler). On normal shutdown, the server would close the pipe, then join the background thread, leading to a race: the background thread might read from the pipe again, getting a "bad fd" error. The fix is simple; just join the thread before closing the pipe.

Closes #12037 from lidavidm/arrow-15181

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>