JCBC-196: Allow Append/Prepend without CAS

To align the append and prepend commands with the others, this
changeset adds method signatures to be used without the CAS
value. This is done by passing 0 as the CAS to the others down
the stack making it essentially ignore it.

Change-Id: I5e5e4f9b0a0853f768a3446107107e8b99b4e847
Reviewed-on: http://review.couchbase.org/26712
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>