Checking if the executor pool is shutdown before executing

[notthetup]

ConnectionHandler uses SingleThreadExecutor to send messages on a given Connector. If the Connector stops, it can cause the ExecutorPool to be stopped. So we need to check if the Executor has not stopped before trying to send something on the connector.

[mchitre]

What was the symptom that this fixes?

[notthetup]

This was happening when a SlaveContainer stopped responding and the connection on the MasterContainer closed. If some Agent was still trying to send a response to some request from the SlaveContainer, it could cause an exception when sending.

1679368369915|INFO|org.arl.fjage.remote.MasterContainer@47:connectionClosed|Connection tcp:///127.0.1.1:1100//127.0.0.1.37286 closed
1679368369941|SEVERE|org.arl.unet.nodeinfo.NodeInfo/audio@44:run|Exception in agent: node

java.util.concurrent.RejectedExecutionException: Task org.arl.fjage.remote.ConnectionHandler$$Lambda$236/1911266333@50ecfb8c rejected from java.util.concurrent.ThreadPoolExecutor@50a52d2b[Shutting down, pool size = 1, active threads = 1, queued tasks = 1, completed tasks = 2]
Stack trace: ...
   org.arl.fjage.remote.ConnectionHandler.printlnQueued(ConnectionHandler.java:229)
   org.arl.fjage.remote.MasterContainer.send(MasterContainer.java:235)
   org.arl.fjage.remote.MasterContainer.send(MasterContainer.java:216)
   org.arl.fjage.Agent.send(Agent.java:411)
    ...