restore classLoaderMatcher to make instrumentation safer

dd-java-agent/instrumentation/servlet-2/servlet-2.gradle

@@ -2,9 +2,11 @@ muzzle {
   pass {
     group = "javax.servlet"
     module = "servlet-api"
-    versions = "[2.3, 2.5]"
+    versions = "[2.3, 3.0)"
     assertInverse = true
   }
+  // can't add a fail block for servlet 3, because servlet 3 is backward compatible with servlet 2.3+,
+  // meaning that for every class that exists in servlet 2, it also exists in servlet 3
 }
 
 apply from: "${rootDir}/gradle/java.gradle"

dd-java-agent/instrumentation/servlet-2/src/main/java/datadog/trace/instrumentation/servlet2/AbstractServlet2Instrumentation.java

@@ -1,6 +1,10 @@
 package datadog.trace.instrumentation.servlet2;
 
+import static datadog.trace.agent.tooling.ClassLoaderMatcher.classLoaderHasClasses;
+import static net.bytebuddy.matcher.ElementMatchers.not;
+
 import datadog.trace.agent.tooling.Instrumenter;
+import net.bytebuddy.matcher.ElementMatcher;
 
 public abstract class AbstractServlet2Instrumentation extends Instrumenter.Default {
 
@@ -8,6 +12,12 @@ public abstract class AbstractServlet2Instrumentation extends Instrumenter.Defau
     super("servlet", "servlet-2");
   }
 
+  // this is required to make sure servlet 2 instrumentation won't apply to servlet 3
+  @Override
+  public ElementMatcher<ClassLoader> classLoaderMatcher() {
+    return not(classLoaderHasClasses("javax.servlet.AsyncEvent", "javax.servlet.AsyncListener"));
+  }
+
   @Override
   public String[] helperClassNames() {
     return new String[] {