Introduce 10 KB threshold for async compression

due to the extremely large performance cost of instantiating AsyncTasks, it's usually not worth bothering with async compression except for very large packets.
While this large overhead can be significantly reduced by using specialized threads, it's early days in the testing stages for such improvements, and for now, we still have this to deal with.

Since async compression is always used prior to player spawn, this change may slightly improve the performance of the pre-join stage of the game.

resources/pocketmine.yml

@@ -85,8 +85,11 @@ network:
   batch-threshold: 256
   #Compression level used when sending batched packets. Higher = more CPU, less bandwidth usage
   compression-level: 6
-  #Use AsyncTasks for compression. Adds half/one tick delay, less CPU load on main thread
+  #Use AsyncTasks for compression during the main game session. Increases latency, but may reduce main thread load
   async-compression: false
+  #Threshold for async compression, in bytes. Only packets larger than this will be compressed asynchronously
+  #Due to large overhead of AsyncTask, async compression isn't worth it except for large packets
+  async-compression-threshold: 10000
   #Experimental. Use UPnP to automatically port forward
   upnp-forwarding: false
   #Maximum size in bytes of packets sent over the network (default 1492 bytes). Packets larger than this will be