Simplify compressed stream implementation a bit.

* subversion/libsvn_subr/stream.c
  (zbaton): Replace read/write/close and subbaton members with pointer to
   substream itself.
  (read_helper_gz): Accept pointer to substream and use svn_stream_read2()
   for reading data from stream instead of calling READ_FN directly.
  (read_handler_gz): Update calls to read_helper_gz().
  (write_handler_gz, close_handler_gz): Use svn_stream_write() and 
   svn_stream_close() instead of calling WRITE_FN and CLOSE_FN directly.
  (svn_stream_compressed): Initialize SUBSTREAM baton member.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1634563 13f79535-47bb-0310-9956-ffa450edef68