Completes 1 of 2 items from puma#1964

This commit adds an endpoint to the status app to print thread
backtraces, and control cli command to call that endpoint.

I tried this locally by starting a server with:

```sh
bundle exec bin/puma test/rackup/hello.ru \
  --control-url="unix://test.sock" \
  --control-token="token"
```

and then printing the backtraces with:

```sh
bundle exec bin/pumactl thread-backtraces \
  --control-url="unix://test.sock" \
  --control-token="token"
```

With this commit the status app sends the thread backtraces as an array
of objects with `name` and `backtrace` keys, rather than as a string
matching the SIGINFO output.

While working on this I noticed that we logged the thread TID twice.
This commit simplifies that so we only log the thread TID once, with
both the label (I don't know when the label would get set) and name if
they are available.