Shorten ThreadPool trimmer and reaper thread names

[joshuay03]

Description

Similar to #2733

Here's an example htop output for 2 workers and 2 threads:

Notice the two puma srv thread names which are really puma srv threadpool trimmer and puma srv threadpool reaper. This PR simply shortens the names to keep them under 15 chars while keeping the naming consistent with the main thread pool threads (e.g. puma srv tp 001).

Note: Puma::Server#run (which initializes a Threadpool) is only every initialised with srv and ctl so this new length works in both cases.

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