World: allow configuring blocks-per-tick for random updating

this makes it much easier to observe and debug stuff that depends on it, such as grass, crop and tree growth, since they'll happen much faster.
A future improvement would be to have the update function use a non-global random so that the output can be reproduced using a given seed.

resources/pocketmine.yml

@@ -120,6 +120,9 @@ chunk-ticking:
   per-tick: 40
   #Radius of chunks around a player to tick
   tick-radius: 3
+  #Number of blocks inside ticking areas' subchunks that get ticked every tick. Higher values will accelerate events
+  #like tree and plant growth, but at a higher performance cost.
+  blocks-per-subchunk-per-tick: 3
   #IDs of blocks not to perform random ticking on.
   disable-block-ticking:
     #- 2 # grass