[SPARK-3139] Made ContextCleaner to not block on shuffles

As a workaround for SPARK-3015, the ContextCleaner was made "blocking", that is, it cleaned items one-by-one. But shuffles can take a long time to be deleted. Given that the RC for 1.1 is imminent, this PR makes a narrow change in the context cleaner - not wait for shuffle cleanups to complete. Also it changes the error messages on failure to delete to be milder warnings, as exceptions in the delete code path for one item does not really stop the actual functioning of the system.
I like this idea.  Note that this reverses the effect of that variable, so the default should now be `false` (since we block for all cleaning operations by default but _not_ for shuffles).