Re: [PATCH] Call RelationDropStorage() for broader range of object drops.

To provide more context, in cstore_fdw creating the storage is easy, we
only need to hook into CREATE FOREIGN TABLE using event triggers. Removing
the storage is not that easy, for DROP FOREIGN TABLE we can use event
triggers. But when we do DROP EXTENSION, the event triggers don't get fired
(because they have already been dropped), so to handle DROP EXTENSION, we
need to hook into the process utility hook. Now to implement this, (1) we
get a list of all cstore tables (2) call postgres's utility hook, (3) if #2
succeeds clean-up all cstore table storage's. But when #3 happens the
relation isn't there anymore, so we create a pseudo-relation [1] and call
RelationDropStorage().

Implementing all of this seemed messy, so we thought maybe postgres could
try to clean-up storage for every relation it assigns a relfilenode for. We
are open to ideas.

[1]
https://github.com/citusdata/cstore_fdw/blob/store_data_in_internal_storage/cstore_fdw.c#L907