Await completion in ServletHttpHandler.service for non-async containers

This patch only returns from service once the request has actually completed, and the response is guaranteed to be available. This fixes micronaut-projects/micronaut-starter#1734 . There may be adverse impact, I'm not sure.

servlet-core/src/main/java/io/micronaut/servlet/http/ServletHttpHandler.java

@@ -66,6 +66,8 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
 
@@ -211,8 +213,22 @@ public void service(ServletExchange<REQ, RES> exchange) {
                     }));
             });
         } else {
+            CompletableFuture<?> termination = new CompletableFuture<>();
             lc.handleNormal()
-                .onComplete((response, throwable) -> onComplete(exchange, req, response, throwable, requestTerminated));
+                .onComplete((response, throwable) -> {
+                    try {
+                        onComplete(exchange, req, response, throwable, requestTerminated);
+                    } finally {
+                        termination.complete(null);
+                    }
+                });
+            try {
+                termination.get();
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            } catch (ExecutionException e) {
+                throw new AssertionError("we only call complete, shouldn't happen", e);
+            }
         }
     }