This commit doesn't do much on its own, but adds a new Java Cleaner API
that lets us hook into Garbage Collector events to reclaim pooled objects and
return them to the pool.
Adds framework for Network Packets to know when a packet has finished dispatching
to get an idea when a packet is done sending to players.
Rewrites PooledObjects impl to properly respect max pool size and remove
almost all risk of contention.
Bumps the Paper Async Task Queue to use 2 threads, and properly shuts it down on shutdown.
Now tracks the full startup time for "Done" message at end, as apparently
Vanillas was done in a place that skipped tracking a lot of code too.
This fixes an issue with ViaVersion
Will now run those tasks just before we print "Done" so that startup
time is appropriately accounted for a plugin, as well as will no longer
trip the watchdog on startup.
Any plugin that tries to bypass this is just going to then trip watchdog
on Spigot too, so don't you dare.
Stop trying to cheat the delay your plugin added to startup time.
This isn't a behavior change because the first thing the tick does....
was run these tasks....
So it's just moving it slightly a few lines to be before a watchdog tick and
to account for it in "Done" time.
Fixes#3294
