pgsql: Preserve clustered index after rewrites with ALTER TABLE

Preserve clustered index after rewrites with ALTER TABLE

A table rewritten by ALTER TABLE would lose tracking of an index usable
for CLUSTER.  This setting is tracked by pg_index.indisclustered and is
controlled by ALTER TABLE, so some extra work was needed to restore it
properly.  Note that ALTER TABLE only marks the index that can be used
for clustering, and does not do the actual operation.

Author: Amit Langote, Justin Pryzby
Reviewed-by: Ibrar Ahmed, Michael Paquier
Discussion: https://postgr.es/m/20200202161718.GI13621@telsasoft.com
Backpatch-through: 9.5

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3f77a670241d114eb4b65ad6eed52298d28dcef8

Modified Files
--------------
src/backend/commands/tablecmds.c          | 46 +++++++++++++++++++++++++++++
src/backend/utils/cache/lsyscache.c       | 23 +++++++++++++++
src/include/utils/lsyscache.h             |  1 +
src/test/regress/expected/alter_table.out | 48 +++++++++++++++++++++++++++++++
src/test/regress/sql/alter_table.sql      | 25 ++++++++++++++++
5 files changed, 143 insertions(+)