Re-implemented chunk sending (#304)

Re-implement chunk sending, send chunks inside a radius instead of below a count

This sends chunks in concentric squares around players. When the radius is hit, it will pad out the radius until a full circle of chunks is loaded around the player.
TODO: implement radius-per-tick, send chunks in concentric circles, use radius for player spawning.

To set your server chunk radius, change `view-distance` in server.properties. Values are intended to be the same as MCPE render distance values. With matching client and server render distances the chunks should reach the horizon.

NOTE: You may notice significantly increased memory usage per player when increasing these values to something respectable. This is normal and expected.
A player with render distance 14 for example will cause loading of 600+ chunks. A player cannot however exceed the render distance limit set in server.properties - the server will simply not send any more chunks.

Render distance of 8 chunks is approximately 200 chunks. This is roughly equivalent to the original default max-chunks of 192 in pocketmine.yml, but sent in a circle instead of a square.

Wait for client to request a chunk radius before ordering chunks

Use 8 for default maximum radius (roughly matches old setting of 192)

Calculate spawn chunk count from chunk-sending.spawn-radius

src/pocketmine/Server.php

@@ -333,8 +333,19 @@ class Server{
 	/**
 	 * @return int
 	 */
-	public function getViewDistance(){
-		return max(56, $this->getProperty("chunk-sending.max-chunks", 256));
+	public function getViewDistance() : int{
+		return max(2, $this->getConfigInt("view-distance", 8));
+	}
+
+	/**
+	 * Returns a view distance up to the currently-allowed limit.
+	 *
+	 * @param int $distance
+	 *
+	 * @return int
+	 */
+	public function getAllowedViewDistance(int $distance) : int{
+		return max(2, min($distance, $this->memoryManager->getViewDistance($this->getViewDistance())));
 	}
 
 	/**
@@ -1384,6 +1395,7 @@ class Server{
 				"enable-rcon" => false,
 				"rcon.password" => substr(base64_encode(random_bytes(20)), 3, 10),
 				"auto-save" => true,
+				"view-distance" => 8
 			]);
 
 			$this->forceLanguage = $this->getProperty("settings.force-language", false);