Assorted message fixes and improvements

contrib/test_decoding/expected/ddl.out

src/backend/access/heap/rewriteheap.c

src/backend/commands/dbcommands.c

src/backend/commands/matview.c

src/backend/commands/tablecmds.c

src/backend/executor/nodeWindowAgg.c

src/backend/replication/logical/reorderbuffer.c

src/backend/replication/logical/snapbuild.c

src/backend/replication/slot.c

src/backend/storage/ipc/dsm_impl.c

src/test/regress/expected/matview.out