[SPARK-6132] ContextCleaner race condition across SparkContexts

The problem is that `ContextCleaner` may clean variables that belong to a different `SparkContext`. This can happen if the `SparkContext` to which the cleaner belongs stops, and a new one is started immediately afterwards in the same JVM. In this case, if the cleaner is in the middle of cleaning a broadcast, for instance, it will do so through `SparkEnv.get.blockManager`, which could be one that belongs to a different `SparkContext`.

JoshRosen and I suspect that this is the cause of many flaky tests, most notably the `JavaAPISuite`. We were able to reproduce the failure locally (though it is not deterministic and very hard to reproduce).

Author: Andrew Or <andrew@databricks.com>

Closes #4869 from andrewor14/cleaner-masquerade and squashes the following commits:

29168c0 [Andrew Or] Synchronize ContextCleaner stop