[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.