JMXUtil.getJmxConnector should retry connection attempts

We previously added a JMXUtil class that makes it easy for dtests to get a JMX connection. It ends up that occasionally the JMX server side needs more time to start up (especially when stopping and restarting instances, which we’re now doing more frequently, or when stopping and restarting the whole cluster). In these cases, JMXUtil.getJmxConnector can fail, when it would be possible to connect if a retry mechanism was added. We should add this capability to the in-jvm dtest framework.