[1.9.4_0608] spam error, then crash

[cwken10]

[issue]
i tried 1.9.4_0608 in 2 players stress test is ok, but test few players join will spam error then crash
http://pastebin.com/s11w8df4

[details]
my partner's machine is double processor G6 X5670, now using have 8 cores, running default spigot.jar maximum cannot over 200% cpu load

[softpak]

I am still try to investigate these similar issues.
It cauese by the chunk system.
PaperSpigot is stable relevantly.
Maybe I will rewirte the whole chunk system for muilti-threading.