cbepctl to support setting of max_cas

A vbucket's max_cas can be "poisoned" by a node with a future clock, this can only be undone by waiting for time to pass or changing a nodes real clock, neither may be desirable if the future clock was a big mistake (think time -s "3016 3-Oct 14:14")

So update epctl so that the damage can be undone to some extent, another tool for the toolbox...