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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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/45ed482f1a899dbf5b0db13686d817718ae8cf97

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(-)