Update Netty to 4.1.51

Minecraft's Netty version was severely out of date. There has been
numerous security fixes, bug fixes, and likely performance fixes
since the version Minecraft uses (4.1.25).

This fixes some known issues with "Closed Channel" spam.

Fixes #3388

Spigot-Server-Patches/0501-Implement-Mob-Goal-API.patch

@@ -5,10 +5,10 @@ Subject: [PATCH] Implement Mob Goal API
 
 
 diff --git a/pom.xml b/pom.xml
-index 60bf6dd28527f6ffcb59779ec9546030245724ca..1ae57e8ab2681e674f97ef3220c5cbf9143c6a7e 100644
+index 389fdcf45eb1513bd6f7f1f0974ace6c00976c2c..84c54a32af54f8460da3257f4423dfcee558e171 100644
 --- a/pom.xml
 +++ b/pom.xml
-@@ -128,6 +128,13 @@
+@@ -133,6 +133,13 @@
              <version>1.3</version>
              <scope>test</scope>
          </dependency>