[SPARK-4055][MLlib] Inconsistent spelling 'MLlib' and 'MLLib'

Thare are some inconsistent spellings 'MLlib' and 'MLLib' in some documents and source codes.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2903 from sarutak/SPARK-4055 and squashes the following commits:

b031640 [Kousuke Saruta] Fixed inconsistent spelling "MLlib and MLLib"
[SPARK-4006] In long running contexts, we encountered the situation of double registe...

...r without a remove in between. The cause for that is unknown, and assumed a temp network issue.

However, since the second register is with a BlockManagerId on a different port, blockManagerInfo.contains() returns false, while blockManagerIdByExecutor returns Some. This inconsistency is caught in a conditional statement that does System.exit(1), which is a huge robustness issue for us.

The fix - simply remove the old id from both maps during register when this happens. We are mimicking the behavior of expireDeadHosts(), by doing local cleanup of the maps before trying to add new ones.

Also - added some logging for register and unregister.

This is just like https://github.com/apache/spark/pull/2854 except it's on master

Author: Tal Sliwowicz <tal.s@taboola.com>

Closes #2886 from tsliwowicz/master-block-mgr-removal and squashes the following commits:

094d508 [Tal Sliwowicz] some more white space change undone
41a2217 [Tal Sliwowicz] some more whitspaces change undone
7bcfc3d [Tal Sliwowicz] whitspaces fix
df9d98f [Tal Sliwowicz] Code review comments fixed
f48bce9 [Tal Sliwowicz] In long running contexts, we encountered the situation of double register without a remove in between. The cause for that is unknown, and assumed a temp network issue.
specify unidocGenjavadocVersion of 0.8

Fixes an issue with being too strict generating javadoc causing errors.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #2893 from holdenk/SPARK-3359-sbtunidoc-java8 and squashes the following commits:

9379a70 [Holden Karau] specify unidocGenjavadocVersion of 0.8
Revert "[SPARK-3812] [BUILD] Adapt maven build to publish effective pom."

This reverts commit c5882c663e054adcd3ecd9f11e91a1929dbc14a3.

I am reverting this becuase it appears to cause the maven tests
to hang.
[SPARK-4019] [SPARK-3740] Fix MapStatus compression bug that could lead to empty results or Snappy errors

This commit fixes a bug in MapStatus that could cause jobs to wrongly return
empty results if those jobs contained stages with more than 2000 partitions
where most of those partitions were empty.

For jobs with > 2000 partitions, MapStatus uses HighlyCompressedMapStatus,
which only stores the average size of blocks.  If the average block size is
zero, then this will cause all blocks to be reported as empty, causing
BlockFetcherIterator to mistakenly skip them.

For example, this would return an empty result:

    sc.makeRDD(0 until 10, 1000).repartition(2001).collect()

This can also lead to deserialization errors (e.g. Snappy decoding errors)
for jobs with > 2000 partitions where the average block size is non-zero but
there is at least one empty block.  In this case, the BlockFetcher attempts to
fetch empty blocks and fails when trying to deserialize them.

The root problem here is that MapStatus has a (previously undocumented)
correctness property that was violated by HighlyCompressedMapStatus:

    If a block is non-empty, then getSizeForBlock must be non-zero.

I fixed this by modifying HighlyCompressedMapStatus to store the average size
of _non-empty_ blocks and to use a compressed bitmap to track which blocks are
empty.

I also removed a test which was broken as originally written: it attempted
to check that HighlyCompressedMapStatus's size estimation error was < 10%,
but this was broken because HighlyCompressedMapStatus is only used for map
statuses with > 2000 partitions, but the test only created 50.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #2866 from JoshRosen/spark-4019 and squashes the following commits:

fc8b490 [Josh Rosen] Roll back hashset change, which didn't improve performance.
5faa0a4 [Josh Rosen] Incorporate review feedback
c8b8cae [Josh Rosen] Two performance fixes:
3b892dd [Josh Rosen] Address Reynold's review comments
ba2e71c [Josh Rosen] Add missing newline
609407d [Josh Rosen] Use Roaring Bitmap to track non-empty blocks.
c23897a [Josh Rosen] Use sets when comparing collect() results
91276a3 [Josh Rosen] [SPARK-4019] Fix MapStatus compression bug that could lead to empty results.
[SPARK-3993] [PySpark] fix bug while reuse worker after take()

After take(), maybe there are some garbage left in the socket, then next task assigned to this worker will hang because of corrupted data.

We should make sure the socket is clean before reuse it, write END_OF_STREAM at the end, and check it after read out all result from python.

Author: Davies Liu <davies.liu@gmail.com>
Author: Davies Liu <davies@databricks.com>

Closes #2838 from davies/fix_reuse and squashes the following commits:

8872914 [Davies Liu] fix tests
660875b [Davies Liu] fix bug while reuse worker after take()
Merge branch 'master' of git://git.apache.org/spark into spark-sql-like-match-modification
Refactored stringOperations.scala

Added test cases to ExpressionEvaluationSuite