Add versioning to pg_dist_partition

There are certain changes that we cannot make to Citus because they break existing distributed tables. A simple example is #1665, which changes shard naming, but there are some bigger features on the horizon that might require breaking changes such as background rebalancing, or distributed snapshot isolation.

It would be useful if we added some versioning scheme to pg_dist_partition such that we could tell whether or not a distributed table follows as .