Implemented network encryption (#2343)

For those who fuss about performance, you can disable the `network.enable-encryption` option to use sessions without encryption.

composer.json

@@ -9,8 +9,10 @@
       "php-64bit": "*",
       "ext-bcmath": "*",
       "ext-curl": "*",
+      "ext-crypto": "^0.3.1",
       "ext-ctype": "*",
       "ext-date": "*",
+      "ext-gmp": "*",
       "ext-hash": "*",
       "ext-json": "*",
       "ext-mbstring": "*",
@@ -24,6 +26,7 @@
       "ext-yaml": ">=2.0.0",
       "ext-zip": "*",
       "ext-zlib": ">=1.2.11",
+      "mdanter/ecc": "^0.5.0",
       "pocketmine/raklib": "^0.12.0",
       "pocketmine/spl": "^0.3.0",
       "pocketmine/binaryutils": "^0.1.0",