[MC-128163] Worldgen worker thread crash

When generating terrain, the Worldgen worker threads crash with Out of memory errors. This is especially likely to happen while generating Jungle biomes.

The crashing of the worker threads is severe. When it happens, the game stops responding to attempts to save the game. Pressing Save and Exit causes the game to hang without doing anything and the game must be terminated externally (eg with Task Manager).

Some examples of crashes from the log:

Caught exception in thread Thread[WorldGen-Worker-93,5,main]
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:714)
	at java.util.concurrent.ForkJoinPool.tryAddWorker(ForkJoinPool.java:1338)
	at java.util.concurrent.ForkJoinPool.deregisterWorker(ForkJoinPool.java:1460)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:167)
	
Caught exception in thread Thread[WorldGen-Worker-46,5,main]
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:714)
	at java.util.concurrent.ForkJoinPool.tryAddWorker(ForkJoinPool.java:1338)
	at java.util.concurrent.ForkJoinPool.deregisterWorker(ForkJoinPool.java:1460)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:167)

Caught exception in thread Thread[WorldGen-Worker-29,5,main]
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:714)
	at java.util.concurrent.ForkJoinPool.tryAddWorker(ForkJoinPool.java:1338)
	at java.util.concurrent.ForkJoinPool.deregisterWorker(ForkJoinPool.java:1460)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:167)

Worldgen worker thread crash - Out of Memory error