features/compress: Compression/DeCompression translator

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>
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.

Other minor changes :
* Check for min file size and buffer size while loading xlator
* Change OS type in gzip header from invalid(254) to UNIX(3)