Fix Unknown QueuedChunk drop error when a chunk is not found during player movement.

Fixes #803, #815, #943, #960

Many users have reported the following error :

java.lang.IllegalStateException: Unknown
org.bukkit.craftbukkit.v1_6_R3.chunkio.QueuedChunk

Until I can pinpoint the exact cause for this error, I have added a temporary
workaround which will avoid a server crash and print debug information for any
chunk not found in the queue when the AsynchronousExecutor attempts to drop it.