pgsql: Relax overly strict assertion

Relax overly strict assertion

Ever since its birth, ReorderBufferBuildTupleCidHash() has contained an
assertion that a catalog tuple cannot change Cmax after acquiring one. But
that's wrong: if a subtransaction executes DDL that affects that catalog
tuple, and later aborts and another DDL affects the same tuple, it will
change Cmax. Relax the assertion to merely verify that the Cmax remains
valid and monotonically increasing, instead.

Add a test that tickles the relevant code.

Diagnosed by, and initial patch submitted by: Arseny Sher
Co-authored-by: Arseny Sher
Discussion: https://postgr.es/m/874l9p8hyw.fsf@ars-thinkpad

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8c67d29fd51c0381d8bce41d35d7726725924616

Modified Files
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contrib/test_decoding/expected/ddl.out | 18 ++++++++++++++++++
contrib/test_decoding/sql/ddl.sql | 13 +++++++++++++
src/backend/replication/logical/reorderbuffer.c | 14 +++++++++-----
3 files changed, 40 insertions(+), 5 deletions(-)