pgstat: Prepare to use mechanism for truncated rels also for droppped rels.

pgstat: Prepare to use mechanism for truncated rels also for droppped rels.

The upcoming shared memory stats patch drops stats for dropped objects in a
transactional manner, rather than removing them later as part of vacuum. This
means that stats for DROP inside a transaction needs to handle aborted
(sub-)transactions similar to TRUNCATE: The stats up to the DROP should be
restored.

Rename the existing infrastructure in preparation.

Author: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/20210405092914[email protected]