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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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f5f9a760a3bc2be95f8716e476150ce128efa22b

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(-)