Logging loop after SIGHUP when a StreamHandler is configured

[matrixbot]

This issue has been migrated from #6550.

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If you use the default logging config, which has a StreamHandler pointing at sys.stderr, and do not set --no-redirect-stdio, then when you send synapse a SIGHUP, it will stack overflow and abort.

The problem appears to be that when you SIGHUP it, the streamhandler gets repointed at the redirected stderr, which goes back into a logger: so as soon as anything is logged, we get an infinite loop.