[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.
Yeah, I am not concerned about the efficiency. I am only concerned about ease of debugging; whether it will be easier to debug if this is exposed as an error or empty batch. I dont have a very strong opinion here, so either is fine.