[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.