Add support for streaming decompression of gzip text

Compressed text formats currently require entire compressed
files be read into memory to be decompressed in a single call
to the decompression codec. This changes the HdfsTextScanner
to drive gzip in a streaming mode, i.e. produce partial output
as input is consumed.

Change-Id: Id5c0805e18cf6b606bcf27a5df4b5f58895809fd
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5233
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins