[SPARK-2890][SQL] Allow reading of data when case insensitive resolution could cause possible ambiguity.

Throwing an error in the constructor makes it possible to run queries, even when there is no actual ambiguity.  Remove this check in favor of throwing an error in analysis when they query is actually is ambiguous.

Also took the opportunity to add test cases that would have caught a subtle bug in my first attempt at fixing this and refactor some other test code.

Author: Michael Armbrust <michael@databricks.com>

Closes #2209 from marmbrus/sameNameStruct and squashes the following commits:

729cca4 [Michael Armbrust] Better tests.
a003aeb [Michael Armbrust] Remove error (it'll be caught in analysis).
[SPARK-2314][SQL] Override collect and take in python library, and count in java library, with optimized versions.

SchemaRDD overrides RDD functions, including collect, count, and take, with optimized versions making use of the query optimizer.  The java and python interface classes wrapping SchemaRDD need to ensure the optimized versions are called as well.  This patch overrides relevant calls in the python and java interfaces with optimized versions.

Adds a new Row serialization pathway between python and java, based on JList[Array[Byte]] versus the existing RDD[Array[Byte]]. I wasn’t overjoyed about doing this, but I noticed that some QueryPlans implement optimizations in executeCollect(), which outputs an Array[Row] rather than the typical RDD[Row] that can be shipped to python using the existing serialization code. To me it made sense to ship the Array[Row] over to python directly instead of converting it back to an RDD[Row] just for the purpose of sending the Rows to python using the existing serialization code.

Author: Aaron Staple <aaron.staple@gmail.com>

Closes #1592 from staple/SPARK-2314 and squashes the following commits:

89ff550 [Aaron Staple] Merge with master.
6bb7b6c [Aaron Staple] Fix typo.
b56d0ac [Aaron Staple] [SPARK-2314][SQL] Override count in JavaSchemaRDD, forwarding to SchemaRDD's count.
0fc9d40 [Aaron Staple] Fix comment typos.
f03cdfa [Aaron Staple] [SPARK-2314][SQL] Override collect and take in sql.py, forwarding to SchemaRDD's collect.
[Docs] minor punctuation fix

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

Closes #2414 from nchammas/patch-1 and squashes the following commits:

14664bf [Nicholas Chammas] [Docs] minor punctuation fix
[SQL][DOCS] Improve section on thrift-server

Taken from liancheng's updates. Merged conflicts with #2316.

Author: Michael Armbrust <michael@databricks.com>

Closes #2384 from marmbrus/sqlDocUpdate and squashes the following commits:

2db6319 [Michael Armbrust] @liancheng's updates