SLLS-70 Close the message processor at exit time

This way we are sure that communication remains possible until the end

src/main/java/org/sonarsource/sonarlint/ls/SonarLintLanguageServer.java

@@ -271,8 +271,6 @@ private static TextDocumentSyncOptions getTextDocumentSyncOptions() {
   @Override
   public CompletableFuture<Object> shutdown() {
     List.<Runnable>of(
-      // start by not processing any more messages from the client
-      () -> Utils.shutdownAndAwait(threadPool, true),
       // prevent creation of new engines
       enginesFactory::shutdown,
       analysisScheduler::shutdown,
@@ -301,6 +299,7 @@ private void invokeQuietly(Runnable call) {
 
   @Override
   public void exit() {
+    invokeQuietly(() -> Utils.shutdownAndAwait(threadPool, true));
     // The Socket will be closed by the client, and so remaining threads will die and the JVM will terminate
   }