def cas!(key, ttl=nil, options=nil, &block)
      cas_core(key, true, ttl, options, &block)
    end
def cas!(key, ttl=nil, options=nil, &block)
      cas_core(key, true, ttl, options, &block)
    end
like #cas, but will yield to the block whether or not the value
 already exists.

 Returns:
 - false if the value was changed by someone else.
 - true if the value was successfully updated.