organize "Javac server event loop" threads in pool and wait for their termination

By default Netty just creates threads light and right and abandons them (see io.netty.util.concurrent.ThreadPerTaskExecutor WTF?)

jps/jps-builders/src/org/jetbrains/jps/javac/ExternalJavacManager.java

@@ -50,6 +50,7 @@ import java.io.File;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.*;
+import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
 
@@ -72,6 +73,7 @@ public class ExternalJavacManager {
   private final Map<UUID, JavacProcessDescriptor> myMessageHandlers = new HashMap<>();
   private int myListenPort = DEFAULT_SERVER_PORT;
   private final Set<ProcessHandler> myRunningHandlers = ContainerUtil.newConcurrentSet();
+  private final ThreadPoolExecutor myExecutor = ConcurrencyUtil.newSingleThreadExecutor("Javac server event loop pool");
 
   public ExternalJavacManager(@NotNull final File workingDir) {
     myWorkingDir = workingDir;
@@ -81,7 +83,7 @@ public class ExternalJavacManager {
   public void start(int listenPort) {
     final ChannelHandler compilationRequestsHandler = new CompilationRequestsHandler();
     final ServerBootstrap bootstrap = new ServerBootstrap()
-      .group(new NioEventLoopGroup(1, ConcurrencyUtil.newNamedThreadFactory("Javac server event loop")))
+      .group(new NioEventLoopGroup(1, myExecutor))
       .channel(NioServerSocketChannel.class)
       .childOption(ChannelOption.TCP_NODELAY, true)
       .childOption(ChannelOption.SO_KEEPALIVE, true)
@@ -187,6 +189,12 @@ public class ExternalJavacManager {
       if (!handler.waitFor(unit.toMillis(time))) {
         return false;
       }
+    }
+    try {
+      myExecutor.awaitTermination(time, unit);
+    }
+    catch (InterruptedException ignored) {
+
     }
     return true;
   }
@@ -209,6 +217,7 @@ public class ExternalJavacManager {
 
   public void stop() {
     myChannelRegistrar.close().awaitUninterruptibly();
+    myExecutor.shutdownNow();
   }
 
   private ExternalJavacProcessHandler launchExternalJavacProcess(UUID uuid,