cbepctl set_vbucket_param max_cas needs to force disk copy of max_cas to the input value

If using cbepctl to change the max_cas of a vbucket, the process of repair fails to update the max_cas stored on disk, due to the flusher having a conditional update which only writes the largest value to disk.

This means that the 'poisoned' cas repair will revert at warmup.

Note that cbepctl correctly updates the in-memory state and all mutations are timestamped using the fixed value - just warmup reverts.

I suspect we need a special set-vb-state to bypass the conditional and just write the new max_cas to disk.

Conversation with support and this may be required for a 7.1.4 MP (and also a candidate for 7.2? but maybe it can slip to 7.2.1)