Network: Added config to pocketmine.yml to allow changing max MTU size

This might be useful for people with older routers where the connection MTU is actually lower than the router allows.

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "content-hash": "f13679b4c6a569a758646a63936dbd18",
+    "content-hash": "539efc3616f87bdf637798e9ea4b6125",
     "packages": [
         {
             "name": "pocketmine/pocketmine-binaryutils",
@@ -111,12 +111,12 @@
             "source": {
                 "type": "git",
                 "url": "https://github.com/pmmp/RakLib.git",
-                "reference": "998af685be51d6ee8a1e206066ccd96c21450bfd"
+                "reference": "c29bebd2df63680e975d5d129aa25476f3ef86ba"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/pmmp/RakLib/zipball/998af685be51d6ee8a1e206066ccd96c21450bfd",
-                "reference": "998af685be51d6ee8a1e206066ccd96c21450bfd",
+                "url": "https://api.github.com/repos/pmmp/RakLib/zipball/c29bebd2df63680e975d5d129aa25476f3ef86ba",
+                "reference": "c29bebd2df63680e975d5d129aa25476f3ef86ba",
                 "shasum": ""
             },
             "require": {
@@ -141,7 +141,7 @@
                 "source": "https://github.com/pmmp/RakLib/tree/master",
                 "issues": "https://github.com/pmmp/RakLib/issues"
             },
-            "time": "2018-01-16T19:33:21+00:00"
+            "time": "2018-01-18T18:09:59+00:00"
         }
     ],
     "packages-dev": [],