Implement SimpleDataSourceStreamReader for python streaming data source

 SimpleDataSourceStreamReader is a simplified version of the DataStreamReader interface.

1. It doesn’t require developers to reason about data partitioning.
2. It doesn’t require getting the latest offset before reading data.

There are 3 functions that needs to be defined 

1. Read data and return the end offset.

def read(self, start: Offset) -> (Iterator[Tuple], Offset)

2. Read data between start and end offset, this is required for exactly once read.

def read2(self, start: Offset, end: Offset) -> Iterator[Tuple]

3. initial start offset of the streaming query.

def initialOffset() -> dict

Implementation: Wrap the SimpleDataSourceStreamReader instance in a DataSourceStreamReader internally and make the prefetching and caching transparent to the data source developer. The record prefetched in python process will be sent to JVM as arrow record batches.