pgsql: pgstat: Prepare to use mechanism for truncated rels also for dro

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 <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6b9501660c9384476ca9a04918f5cf94379e419e

Modified Files
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src/backend/postmaster/pgstat.c | 93 ++++++++++++++++++++++-------------------
src/include/pgstat.h | 12 +++---
2 files changed, 57 insertions(+), 48 deletions(-)