[SPARK-3912][Streaming] Fixed flakyFlumeStreamSuite

@harishreedharan @pwendell 
See JIRA for diagnosis of the problem 
https://issues.apache.org/jira/browse/SPARK-3912

The solution was to reimplement it. 
1. Find a free port (by binding and releasing a server-scoket), and then use that port
2. Remove thread.sleep()s, instead repeatedly try to create a sender and send data and check whether data was sent. Use eventually() to minimize waiting time.
3. Check whether all the data was received, without caring about batches.


Makes sense. Thanks!