Ensure metadata/state compatibility of Citus before upgrades

Quoting @marcocitus: `maybe we need a citus_prepare_citus_upgrade`.

So, basically before upgrading Citus to a new version, call a UDF, citus_prepare_citus_upgrade which does sanity checks.

For example, we might want to ensure that all nodes have metadata synced before the upgrade. Or, in the past we had thinks like this: https://github.com/citusdata/citus/commit/9e6ca23286f746757ce3aae2a422c8edee06ec02#diff-e8722019c09d4cd7fd7c1d2c99aeb190329e1b7e258ac21e2226575f93570bddR16