[SPARK-3543] Write TaskContext in Java and expose it through a static accessor.

Author: Prashant Sharma <prashant.s@imaginea.com>
Author: Shashank Sharma <shashank21j@gmail.com>

Closes #2425 from ScrapCodes/SPARK-3543/withTaskContext and squashes the following commits:

8ae414c [Shashank Sharma] CR
ee8bd00 [Prashant Sharma] Added internal API in docs comments.
ddb8cbe [Prashant Sharma] Moved setting the thread local to where TaskContext is instantiated.
a7d5e23 [Prashant Sharma] Added doc comments.
edf945e [Prashant Sharma] Code review git add -A
f716fd1 [Prashant Sharma] introduced thread local for getting the task context.
333c7d6 [Prashant Sharma] Translated Task context from scala to java.
Close #2194.
Slaves file is now a template.

Change 0dc868e removed the `conf/slaves` file and made it a template like most of the other configuration files. This means you can no longer run `make-distribution.sh` unless you manually create a slaves file to be statically bundled in your distribution, which seems at odds with making it a template file.

Author: Sarah Gerweck <sarah.a180@gmail.com>

Closes #2549 from sarahgerweck/noMoreSlaves and squashes the following commits:

d11d99a [Sarah Gerweck] Slaves file is now a template.
[Build]remove spark-staging-1030

Since 1.1.0 has published, remove spark-staging-1030.

Author: wangfei <wangfei1@huawei.com>

Closes #2532 from scwf/patch-2 and squashes the following commits:

bc9e00b [wangfei] remove spark-staging-1030
[SQL][DOCS] Clarify that the server is for JDBC and ODBC

Author: Michael Armbrust <michael@databricks.com>

Closes #2527 from marmbrus/patch-1 and squashes the following commits:

a0f9f1c [Michael Armbrust] [SQL][DOCS] Clarify that the server is for JDBC and ODBC
[SPARK-3675][SQL] Allow starting a JDBC server on an existing context

Author: Michael Armbrust <michael@databricks.com>

Closes #2515 from marmbrus/jdbcExistingContext and squashes the following commits:

7866fad [Michael Armbrust] Allows starting a JDBC server on an existing context.
stop, start and destroy require the EC2_REGION

i.e
./spark-ec2 --region=us-west-1 stop yourclustername

Author: Jeff Steinmetz <jeffrey.steinmetz@gmail.com>

Closes #2473 from jeffsteinmetz/master and squashes the following commits:

7491f2c [Jeff Steinmetz] fix case in EC2 cluster setup documentation
bd3d777 [Jeff Steinmetz] standardized ec2 documenation to use <lower-case> sample args
2bf4a57 [Jeff Steinmetz] standardized ec2 documenation to use <lower-case> sample args
68d8372 [Jeff Steinmetz] standardized ec2 documenation to use <lower-case> sample args
d2ab6e2 [Jeff Steinmetz] standardized ec2 documenation to use <lower-case> sample args
520e6dc [Jeff Steinmetz] standardized ec2 documenation to use <lower-case> sample args
37fc876 [Jeff Steinmetz] stop, start and destroy require the EC2_REGION
[SPARK-1021] Defer the data-driven computation of partition bounds in so...

...rtByKey() until evaluation.

Author: Erik Erlandson <eerlands@redhat.com>

Closes #1689 from erikerlandson/spark-1021-pr and squashes the following commits:

50b6da6 [Erik Erlandson] use standard getIteratorSize in countAsync
4e334a9 [Erik Erlandson] exception mystery fixed by fixing bug in ComplexFutureAction
b88b5d4 [Erik Erlandson] tweak async actions to use ComplexFutureAction[T] so they handle RangePartitioner sampling job properly
b2b20e8 [Erik Erlandson] Fix bug in exception passing with ComplexFutureAction[T]
ca8913e [Erik Erlandson] RangePartition sampling job -> FutureAction
7143f97 [Erik Erlandson] [SPARK-1021] modify range bounds variable to be thread safe
ac67195 [Erik Erlandson] [SPARK-1021] Defer the data-driven computation of partition bounds in sortByKey() until evaluation.
[SPARK-3535][Mesos] Fix resource handling.

 - Added fractional with minimum task memory overhead
 - Correctly specify CPUs for executor
 - Actually check if offer is sufficient, rather than just assuming it
   is