[SPARK-3500] [SQL] use JavaSchemaRDD as SchemaRDD._jschema_rdd

Currently, SchemaRDD._jschema_rdd is SchemaRDD, the Scala API (coalesce(), repartition()) can not been called in Python easily, there is no way to specify the implicit parameter `ord`. The _jrdd is an JavaRDD, so _jschema_rdd should also be JavaSchemaRDD.

In this patch, change _schema_rdd to JavaSchemaRDD, also added an assert for it. If some methods are missing from JavaSchemaRDD, then it's called by _schema_rdd.baseSchemaRDD().xxx().

BTW, Do we need JavaSQLContext?

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

Closes #2369 from davies/fix_schemardd and squashes the following commits:

abee159 [Davies Liu] use JavaSchemaRDD as SchemaRDD._jschema_rdd

(cherry picked from commit 885d1621bc06bc1f009c9707c3452eac26baf828)
Signed-off-by: Josh Rosen <joshrosen@apache.org>

Conflicts:
	python/pyspark/tests.py
[SPARK-3515][SQL] Moves test suite setup code to beforeAll rather than in constructor

Please refer to the JIRA ticket for details.

**NOTE** We should check all test suites that do similar initialization-like side effects in their constructors. This PR only fixes `ParquetMetastoreSuite` because it breaks our Jenkins Maven build.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2375 from liancheng/say-no-to-constructor and squashes the following commits:

0ceb75b [Cheng Lian] Moves test suite setup code to beforeAll rather than in constructor

(cherry picked from commit 6d887db7891be643f0131b136e82191b5f6eb407)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SQL] [Docs] typo fixes

* Fixed random typo
* Added in missing description for DecimalType

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2367 from nchammas/patch-1 and squashes the following commits:

aa528be [Nicholas Chammas] doc fix for SQL DecimalType
3247ac1 [Nicholas Chammas] [SQL] [Docs] typo fixes

(cherry picked from commit a523ceaf159733dabcef84c7adc1463546679f65)
Signed-off-by: Michael Armbrust <michael@databricks.com>
SPARK-3039: Allow spark to be built using avro-mapred for hadoop2

SPARK-3039: Adds the maven property "avro.mapred.classifier" to build spark-assembly with avro-mapred with support for the new Hadoop API. Sets this property to hadoop2 for Hadoop 2 profiles.

I am not very familiar with maven, nor do I know whether this potentially breaks something in the hive part of spark. There might be a more elegant way of doing this.

Author: Bertrand Bossy <bertrandbossy@gmail.com>

Closes #1945 from bbossy/SPARK-3039 and squashes the following commits:

c32ce59 [Bertrand Bossy] SPARK-3039: Allow spark to be built using avro-mapred for hadoop2

(cherry picked from commit c243b21a8ba2610266702e00d7d4b5443cb1f687)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-3518] Remove wasted statement in JsonProtocol

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

Closes #2380 from sarutak/SPARK-3518 and squashes the following commits:

8a1464e [Kousuke Saruta] Replaced a variable with simple field reference
c660fbc [Kousuke Saruta] Removed useless statement in JsonProtocol.scala

(cherry picked from commit e59fac1f97c3fbeeb6defd12625a49763a353156)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>