Use smaller default size for thread pools (#94)

fixes #93

modbus-tcp/src/main/java/com/digitalpetri/modbus/tcp/Netty.java

@@ -31,7 +31,7 @@ public Thread newThread(Runnable r) {
         }
       };
 
-      EVENT_LOOP = new NioEventLoopGroup(0, threadFactory);
+      EVENT_LOOP = new NioEventLoopGroup(1, threadFactory);
     }
 
     return EVENT_LOOP;

modbus/src/main/java/com/digitalpetri/modbus/Modbus.java

@@ -90,10 +90,7 @@ public Thread newThread(Runnable r) {
         }
       };
 
-      ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(
-          Runtime.getRuntime().availableProcessors(),
-          threadFactory
-      );
+      var executor = new ScheduledThreadPoolExecutor(1, threadFactory);
 
       executor.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
 

modbus/src/main/java/com/digitalpetri/modbus/TimeoutScheduler.java

@@ -1,5 +1,7 @@
 package com.digitalpetri.modbus;
 
+import com.digitalpetri.modbus.internal.util.ExecutionQueue;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
@@ -23,9 +25,10 @@ interface TimeoutHandle {
 
   }
 
-  static TimeoutScheduler fromScheduledExecutor(ScheduledExecutorService ses) {
+  static TimeoutScheduler create(Executor executor, ScheduledExecutorService scheduledExecutor) {
     return (task, delay, unit) -> {
       final var ref = new AtomicReference<ScheduledFuture<?>>();
+      final ExecutionQueue queue = new ExecutionQueue(executor);
 
       var handle = new TimeoutHandle() {
         @Override
@@ -44,7 +47,12 @@ public boolean isCancelled() {
       };
 
       synchronized (ref) {
-        ref.set(ses.schedule(() -> task.run(handle), delay, unit));
+        ScheduledFuture<?> future = scheduledExecutor.schedule(
+            () -> queue.submit(() -> task.run(handle)),
+            delay,
+            unit
+        );
+        ref.set(future);
       }
 
       return handle;

modbus/src/main/java/com/digitalpetri/modbus/client/ModbusClientConfig.java

@@ -64,7 +64,10 @@ public static class Builder {
      */
     public ModbusClientConfig build() {
       if (timeoutScheduler == null) {
-        timeoutScheduler = TimeoutScheduler.fromScheduledExecutor(Modbus.sharedScheduledExecutor());
+        timeoutScheduler = TimeoutScheduler.create(
+            Modbus.sharedExecutor(),
+            Modbus.sharedScheduledExecutor()
+        );
       }
 
       return new ModbusClientConfig(