Reduced chunk compression level to 7

According to benchmarks 8 and 9 are simply orders of magnitude slower while not saving enough bytes to make it worth the performance loss. 7 is a good balance.

src/pocketmine/level/format/io/ChunkRequestTask.php

@@ -72,7 +72,7 @@ class ChunkRequestTask extends AsyncTask{
 
 		$batch = new BatchPacket();
 		$batch->addPacket($pk);
-		$batch->setCompressionLevel(9);
+		$batch->setCompressionLevel(7);
 		$batch->encode();
 
 		$this->setResult($batch->buffer, false);