Fail fast for certain DDL commands

[jasonmp85]

#1 and #6 discuss issues around dropping shards (or attempting to use DROP TABLE), but there are larger questions around other DDL commands.

• [ ] Discuss whether DDL commands against distributed tables should be allowed at all
• [ ] List any advanced use cases for permitting certain commands
• [ ] Determine feasibility of intercepting certain commands
• [ ] Choose mechanism of interception (utility hook or event trigger)
• [ ] Add code to warn or error when users issue DDL commands against distributed tables