benchmarking(boomer): disentangle dynConfig

[Max Smythe (maxsmythe)]

dynconfig had become one struct of every knob every user class ever needed, with a payload mirror, a merge, a validator, and a log line that all need to grow by hand for each new field, and every class saw and was checked against every other class's knobs.

The payload is now an arbitrary JSON object, and a worker runs exactly one user class, known at launch. So each class declares the keys it reads as a struct with json tags, wraps it in a dynconfig.Typed codec with its defaults and its validation func, and registers the codec with its userclass.Entry. The worker builds the holder from that codec; the holder stores the typed value, folds each payload into it with one decode over the current value (so an absent or null key keeps what it had), refuses a payload the class cannot run on in favor of the last good config, and hands the class its knobs back through dynconfig.Get. Keys the class does not name are ignored. The wait window and the resume and lifecycle modes are shared pieces a class embeds; durations travel as seconds, the unit the locust flags use.

Now we can implement user classes more independently of each other.

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR