Revert of HDFS-12914 breaks the branch-2

[Wei-Chiu Chuang <we...@cloudera.com.INVALID>]

Heads up,

My bad. This is becoming a mess.

For the context, HDFS-12914 in branch-2 has a test failure, so I reverted
it. But HDFS-13898 used a helper method
(BlockManager#setBlockManagerForTesting()) added by HDFS-12914, so the
revert breaks the build.

Here's what I propose:

(1) File a new Jira to add the missing helper method. I don't want to
revert HDFS-13898 <https://issues.apache.org/jira/browse/HDFS-13898> because
ultimately we want to cherry pick HDFS-12914
<https://issues.apache.org/jira/browse/HDFS-12914> into branch-2, and we
still need that missing helper method.

(2) resolve HDFS-12914 since this is already a mess here. The 3.x branches
are good, and I want to 2.x to stay good as well.

(3) I'll file a new Jira to backport HDFS-12914
<https://issues.apache.org/jira/browse/HDFS-12914> to branch-2, later.

This should help bring a little sanity to these branches quickly. Shout out
at me if you have any questions.

Weichiu