This commit introduces the following changes:
1. Introduce the `SignalTrapLogger` singleton which starts a single
thread that polls a queue to log messages with the specified logger.
This thread is necessary becasue most loggers cannot be used inside
the `Signal.trap` context as they rely on mutexes which are not
allowed within the context.
2. Moves the monkey patch in `freedom_patches/unicorn_http_server_patch.rb` to
`config/unicorn.config.rb` which is already monkey patching
`Unicorn::HttpServer`.
3. `Unicorn::HttpServer` will now automatically send a `USR2` signal to
a unicorn worker 2 seconds before the worker is timed out by the
Unicorn master.
4. When a Unicorn worker receives a `USR2` signal, it will now log only
the main thread's backtraces to `Rails.logger`. Previously, it was
`put`ing the backtraces to `STDOUT` which most people wouldn't read.
Logging it via `Rails.logger` will make the backtraces easily
accessible via `/logs`.
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