features/compress: Added writev

* Writev compresses data on client and decompresses on server.
  Similarly readv compresses data on server and decompresses
  on client.

* The possible values for "mode" option of the cdc xlator
  were compress/decompress earlier. This has been changed
  after adding writev. Currently the mode option can be
  client/server.

* Removed gzip header while compressing which was an overhead.
  Gzip header will henceforth be used for testing/debugging only.
  Compressed data is identified by xdata key/value.

* Added a debug xlator option. When this is turned on, compressed
  data will be dumped to disk as a gzip file. This is to be used
  in testing.

Other minor changes :
* Check for min file size and buffer size while loading xlator
* Remove cleanup_out label from fop
* Change OS type from invalid to UNIX in gzip header

Change-Id: If4eb4f8af2d63b4bc059e9daaecfda9e1ad8ea56
Signed-off-by: Prashanth Pai <nullpai@gmail.com>
features/compress: Set compression xlator options from CLI

Ex : gluster volume set <vol_name> compress on
     gluster volume set <vol_name> compress.buffer-size 32768

Change-Id: Id30459fd76f7b5155d7b063c36e9caeee4923339
Signed-off-by: Prashanth Pai <nullpai@gmail.com>