Periodically save block chunk meta

[Maxopoly]

The current setup will never flush out reinforcements and co if the server hard crashes (deadlock or physical restart), which is especially problematic if a lot of changes were done to a chunk and it was never unloaded. We need a time based priority queue to slowly write edited block meta over time to the db.

[Gjum]

(moving from #110 (duplicate))

currently only flushes changes some time after chunk unload, which often only happens on server shutdown, causing loss of hours of data when the server crashes (as happened a few times already)

flushing every 30min/5min/whatever would make crashes less painful, and possibly also shorten shutdown/restart times

shadowtrot said:

it would be better to save async to cut main thread load, but for that it needs updated to be safe to do that async
currently it synchronizes on the input and output queues and the chunkcoord when unloading to make sure main thread doesn't interfere