pgsql: During heap rebuild, lock any TOAST index until end of transacti

During heap rebuild, lock any TOAST index until end of transaction.

swap_relation_files() calls toast_get_valid_index() to find and lock
this index, just before swapping with the rebuilt TOAST index. The
latter function releases the lock before returning. Potential for
mischief is low; a concurrent session can issue ALTER INDEX ... SET
(fillfactor = ...), which is not alarming. Nonetheless, changing
pg_class.relfilenode without a lock is unconventional. Back-patch to
9.5 (all supported versions), because another fix needs this.

Discussion: https://postgr.es/m/20191226001521.GA1772687@rfd.leadboat.com

Branch
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REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/88b3a6cd262344f5de64eab31804a0f39a7c6337

Modified Files
--------------
src/backend/access/heap/tuptoaster.c | 4 ++--
src/backend/commands/cluster.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)