features/compress: Compression/DeCompression translator

Changes made to original code :
* Changed Makefile.am to reflect changes from commit 58e6296f
* Code to be merged into glusterd-volgen.c moved to
  glusterd-volume-set.c file introduced by commit 99fe09f4
* Added .default_value and .description to xlator options[]
  and removed them from glusterd-volume-set.c
* Moved all cdc xlator specific functions to cdc-helper.c
  retaining only fop implementations in cdc.c

The following work has been done by Venky Shankar.
Original-author : Venky Shankar <vshankar@redhat.com>

On the server this translator compresses (deflate) data before
it's pushed out to the network to the client. On the client
this compressed data is decompressed (inflate) to it's original
form. What is saved is the amount of data that is sent over
the wire.

(De)Compression is done using Zlib library (inflate/deflate
routines respectively). As of now, data is transferred in
gzip format (header + <compressed data> + trailer). The
header can be taken away to make data more lightweight.
Trailer is needed as it contains the CRC and length of the
original (uncompressed) data used for validating inflated
data.

HOW TO USE
----------
As of now, you'd need to edit the volfile to use this feature;
there is no way to do so from CLI. On the server, load it above
the server translator so that data is compressed just before it
is send out to the network. On the client, load it below the
client translator. But since there can be multiple instances of
the client translator (distribute, replicate etc..), you'd need
to have that many instances of this translator *pointing* to
it's respective client translator.

Change-Id: Ib7a66b6f1f70fe002b7c513588cdf75c69370805
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Prashanth Pai <nullpai@gmail.com>
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>