[spark-3586][streaming]Support nested directories in Spark Streaming

For text files, the method streamingContext.textFileStream(dataDirectory). 
The improvement of the streaming to Support subdirectories,spark streaming can  monitor the subdirectories dataDirectory and process any files created in that directory.
eg:
streamingContext.textFileStream(/test). 
Look at the direction contents:
/test/file1
/test/file2
/test/dr/file1
if the directory "/test/dr/" have new file "file2" ,spark streaming can process  the file


Nit: you can use string interpolation to make it a little simpler