[SPARK-3015] Block on cleaning tasks to prevent Akka timeouts

More detail on the issue is described in [SPARK-3015](https://issues.apache.org/jira/browse/SPARK-3015), but the TLDR is if we send too many blocking Akka messages that are dependent on each other in quick successions, then we end up causing a few of these messages to time out and ultimately kill the executors. As of #1498, we broadcast each RDD whether or not it is persisted. This means if we create many RDDs (each of which becomes a broadcast) and the driver performs a GC that cleans up all of these broadcast blocks, then we end up sending many `RemoveBroadcast` messages in parallel and trigger the chain of blocking messages at high frequencies.

We do not know of the Akka-level root cause yet, so this is intended to be a temporary solution until we identify the real issue. I have done some preliminary testing of enabling blocking and observed that the queue length remains quite low (< 1000) even under very intensive workloads.

In the long run, we should do something more sophisticated to allow a limited degree of parallelism through batching clean up tasks or processing them in a sliding window. In the longer run, we should clean up the whole `BlockManager*` message passing interface to avoid unnecessarily awaiting on futures created from Akka asks.

tdas pwendell mengxr

Author: Andrew Or <andrewor14@gmail.com>

Closes #1931 from andrewor14/reference-blocking and squashes the following commits:

d0f7195 [Andrew Or] Merge branch 'master' of github.com:apache/spark into reference-blocking
ce9daf5 [Andrew Or] Remove logic for logging queue length
111192a [Andrew Or] Add missing space in log message (minor)
a183b83 [Andrew Or] Switch order of code blocks (minor)
9fd1fe6 [Andrew Or] Remove outdated log
104b366 [Andrew Or] Use the actual reference queue length
0b7e768 [Andrew Or] Block on cleaning tasks by default + log error on queue full
[SPARK-3045] Make Serializer interface Java friendly

Author: Reynold Xin <rxin@apache.org>

Closes #1948 from rxin/kryo and squashes the following commits:

a3a80d8 [Reynold Xin] [SPARK-3046] use executor's class loader as the default serializer classloader
3d13277 [Reynold Xin] Reverted that in TestJavaSerializerImpl too.
196f3dc [Reynold Xin] Ok one more commit to revert the classloader change.
c49b50c [Reynold Xin] Removed JavaSerializer change.
afbf37d [Reynold Xin] Moved the test case also.
a2e693e [Reynold Xin] Removed the Kryo bug fix from this pull request.
c81bd6c [Reynold Xin] Use defaultClassLoader when executing user specified custom registrator.
68f261e [Reynold Xin] Added license check excludes.
0c28179 [Reynold Xin] [SPARK-3045] Make Serializer interface Java friendly [SPARK-3046] Set executor's class loader as the default serializer class loader