[SPARK-14555] First cut of Python API for Structured Streaming

## What changes were proposed in this pull request?

This patch provides a first cut of python APIs for structured streaming. This PR provides the new classes:
 - ContinuousQuery
 - Trigger
 - ProcessingTime
in pyspark under `pyspark.sql.streaming`.

In addition, it contains the new methods added under:
 -  `DataFrameWriter`
     a) `startStream`
     b) `trigger`
     c) `queryName`

 -  `DataFrameReader`
     a) `stream`

 - `DataFrame`
    a) `isStreaming`

This PR doesn't contain all methods exposed for `ContinuousQuery`, for example:
 - `exception`
 - `sourceStatuses`
 - `sinkStatus`

They may be added in a follow up.

This PR also contains some very minor doc fixes in the Scala side.

## How was this patch tested?

Python doc tests

TODO:
 - [ ] verify Python docs look good

Author: Burak Yavuz <brkyvz@gmail.com>
Author: Burak Yavuz <burak@databricks.com>

Closes #12320 from brkyvz/stream-python.

1 files changed, 0 insertions, 15 deletions

diff --git a/python/pyspark/ml/util.py b/python/pyspark/ml/util.py
index 841bfb47e1..7003e587ad 100644
--- a/python/pyspark/ml/util.py
+++ b/python/pyspark/ml/util.py
@@ -17,7 +17,6 @@
 
 import sys
 import uuid
-from functools import wraps
 
 if sys.version > '3':
     basestring = str
@@ -39,20 +38,6 @@ def _jvm():
         raise AttributeError("Cannot load _jvm from SparkContext. Is SparkContext initialized?")
 
 
-def keyword_only(func):
-    """
-    A decorator that forces keyword arguments in the wrapped method
-    and saves actual input keyword arguments in `_input_kwargs`.
-    """
-    @wraps(func)
-    def wrapper(*args, **kwargs):
-        if len(args) > 1:
-            raise TypeError("Method %s forces keyword arguments." % func.__name__)
-        wrapper._input_kwargs = kwargs
-        return func(*args, **kwargs)
-    return wrapper
-
-
 class Identifiable(object):
     """
     Object with a unique ID.