WIP: replace connection pool

[tpolecat]

This replaces the built-in pool with pool-party, which is a lot simpler and has much better diagnostics. We need to hook in a default event listener that at least reports errors.

This also drops support for 2.12 because of annoying issues getting pool-party to compile, but I guess I should look into it a little bit more because it's going to annoy people.

After merging we need to tag an RC to let people test the new pool before a full upgrade, since it's a potentially big problem if it's buggier than the existing pool (I don't think it is but concurrency is hard).

[codecov-commenter]

Codecov Report

Merging #559 (1734184) into main (ae26b0b) will decrease coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #559      +/-   ##
==========================================
- Coverage   85.27%   84.95%   -0.33%     
==========================================
  Files         124      123       -1     
  Lines        1596     1575      -21     
  Branches       34       35       +1     
==========================================
- Hits         1361     1338      -23     
- Misses        235      237       +2     

Impacted Files	Coverage Δ
modules/core/shared/src/main/scala/Session.scala	54.54% <100.00%> (+0.69%)	⬆️
...les/core/shared/src/main/scala/util/Recycler.scala	42.85% <0.00%> (-14.29%)	⬇️

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