Improvements on weak meta data

A note from that pull request for further reference:

There are currently two issues that will be fixed in the future. First is that if a new DC is added during execution, a stable time for this DC might be visible before all nodes have participated. Second is that to ensure all nodes and vnodes have participated or if a vnode has moved from one node to another, each time the stable time is updated, it checks that no changes to the ring have occurred. This incurs O(vnodes) local calculation, when it should be only O(1).