[SPARK-11654][SQL] add reduce to GroupedDataset

[marmbrus]

This PR adds a new method, reduce, to GroupedDataset, which allows similar operations to reduceByKey on a traditional PairRDD.

val ds = Seq("abc", "xyz", "hello").toDS()
ds.groupBy(_.length).reduce(_ + _).collect()  // not actually commutative :P

res0: Array(3 -> "abcxyz", 5 -> "hello")

While implementing this method and its test cases several more deficiencies were found in our encoder handling. Specifically, in order to support positional resolution, named resolution and tuple composition, it is important to keep the unresolved encoder around and to use it when constructing new Datasets with the same object type but different output attributes. We now divide the encoder lifecycle into three phases (that mirror the lifecycle of standard expressions) and have checks at various boundaries:

• Unresoved Encoders: all users facing encoders (those constructed by implicits, static methods, or tuple composition) are unresolved, meaning they have only UnresolvedAttributes for named fields and BoundReferences for fields accessed by ordinal.
• Resolved Encoders: internal to a [Grouped]Dataset the encoder is resolved, meaning all input has been resolved to a specific AttributeReference. Any encoders that are placed into a logical plan for use in object construction should be resolved.
• BoundEncoder: Are constructed by physical plans, right before actual conversion from row -> object is performed.

It is left to future work to add explicit checks for resolution and provide good error messages when it fails. We might also consider enforcing the above constraints in the type system (i.e. fromRow only exists on a ResolvedEncoder), but we should probably wait before spending too much time on this.

[marmbrus]

/cc @cloud-fan

[SparkQA]

Test build #45762 has finished for PR 9673 at commit ab2dbb9.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class TypedColumn[-T, U](\n

[SparkQA]

Test build #45775 has finished for PR 9673 at commit f247497.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class TypedColumn[-T, U](\n

[marmbrus]

I'm going to merge this to master and 1.6. Will address any comments in a follow-up.

[cloud-fan]

Does the input class reference to the T of this Dataset?

[marmbrus]

Yeah, it is. Actually, this comment is stale and there is a test now to show that it works.