Replace copied (from rsync) checksum code by adler32() from zlib

The weak checksum code that is included in libglusterfs has initialy
been copied from the rsync sources. Instead of maintaining a copy of a
function, we should use a function from a shared library. The algorithm
seems to be Adler-32, zlib provides an implementation.

The strong checksum function has already been replaced by MD5 from
OpenSSL. It is time to also remove the comments about the origin of the
implementation, because it is not correct anymore.

Change-Id: I70c16ae1d1c36b458a035e4adb3e51a20afcf652
BUG: 1149943
Reported-by: Wade Mealing <wmealing@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9035
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>