atxs: leaking threads on verification (?) codepath

[dshulyak]

i noticed that amount of threads increases over time, see first screenshot.
it correlates with growth of atxs, so it is almost 100% is on post verification codepath.

i looked up on the internet and found old go issue which explains that go doesn't recycle spawned threads golang/go#14592 .on testnet05 (3rd screenshot) amount of threads is not constant either.