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.
Compressed data is identified by xdata key/value

Other minor changes :
* Check for min file size and buffer size while loading xlator
* Remove cleanup_out label from fop

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>