Use a proper thread pool in ServerLoginPacketListenerImpl and MCUtil

[egg82]

Fixes #8797

An arbitrary-ish number of 1024 was chosen for the bounded queue size.
The asyncExecutor in MCUtil was left unbounded.

[NoahvdAa]

Try to avoid reformatting these large blocks to keep the diff small :)

This should probably also go into the original patch.

[electronicboy]

1. You mangled the formatting
2. core size of 16 is excessive, especially as the thread pool will race towards that needlessly; I'd probably just go for like 4 or something, and maybe make it configurable; ofc, this is actually probably a pretty good target for green threads in the future

[lynxplay]

Are you still planning on moving forward with this ? I don't recall the dhrama discussion about it but I'd close this otherwise.

[lynxplay]

Closing this given it has gone rather stale 😓 (love you egg <3)