Redo chunk unloading

[astei]

Right now, chunk unloading is done quite aggressively: if it's not a spawn chunk, once a chunk has been left by a player the chunk is queued for unload.

This is pretty bad as it puts pressure on the chunk provider (and by extension causes sync I/O). While async chunk loading allows the server to cope better with this, the current situation is not ideal. At the very least the I/O should be offloaded to the existing I/O thread pool.

[astei]

Current ideas for this:

• Add a delay to chunk unloading (minor intensity)
• Async saving (high intensity)