Fixes #585 MessageId not limited to 65535

Message ID should always be in the range 1 - 65535. This adds a test,
and, and resets the counter to 1 when needed.

broker/src/main/java/io/moquette/broker/MQTTConnection.java

@@ -503,7 +503,22 @@ public void resendNotAckedPublishes() {
     }
 
     int nextPacketId() {
-        return lastPacketId.incrementAndGet();
+        final int next = lastPacketId.incrementAndGet();
+        if (next <= 65_535) {
+            return next;
+        }
+        return resetPacketId();
+    }
+
+    private int resetPacketId() {
+        synchronized(lastPacketId) {
+            if (lastPacketId.get() <= 65_535) {
+                // Another thread reset it before us...
+                return lastPacketId.incrementAndGet();
+            }
+            lastPacketId.set(1);
+            return 1;
+        }
     }
 
     @Override

broker/src/test/java/io/moquette/broker/MQTTConnectionConnectTest.java

@@ -319,4 +319,13 @@ public void testForceClientDisconnection_issue116() {
         assertFalse(channel.isOpen(), "First 'FAKE_CLIENT_ID' channel MUST be closed by the broker");
         assertTrue(anotherChannel.isOpen(), "Second 'FAKE_CLIENT_ID' channel MUST be still open");
     }
+
+    @Test
+    public void testMessageIdGeneration() {
+        for (int i = 0; i < 65_536; i++) {
+            int nextPacketId = sut.nextPacketId();
+            assertTrue(nextPacketId > 0, "Packet ID must be > 0");
+            assertTrue(nextPacketId <= 65_535, "Packet ID must be <= 65_535");
+        }
+    }
 }