Prevent deletion of metadata when still in use

[jasonmp85]

Running DROP EXTENSION will remove pg_shard's metadata tables, including information about the placement and health of shards. This behavior should only be possible with the CASCADE modifier: otherwise the extension should refuse to be dropped until all distributed tables have been removed.

Perhaps we can look into the dependency tracking available in PostgreSQL's catalog tables to satisfy this need.

[jasonmp85]

@onderkalaci, in case you pick this up, you might want to look at recordDependencyOnSingleRelExpr and recordDependencyOn, which are helpers for making changes to the pg_depend catalog.