[SPARK-3154][STREAMING] Make FlumePollingInputDStream shutdown cleaner.

Currently lot of errors get thrown from Avro IPC layer when the dstream
or sink is shutdown. This PR cleans it up. Some refactoring is done in the
receiver code to put all of the RPC code into a single Try and just recover
from that. The sink code has also been cleaned up.
Why are there so many nested Try's and try's in this. The code looks pretty complex and verbose. I strongly recommend taking another pass, may be break the code into smaller functions (with good semantically-meaningful names).