pgsql: Formalize WAL record for XLOG_CHECKPOINT_REDO

Formalize WAL record for XLOG_CHECKPOINT_REDO

XLOG_CHECKPOINT_REDO only contains the wal_level copied straight in
without an encapsulating record structure. While it works, it makes
future uses of XLOG_CHECKPOINT_REDO hard as there is nowhere to put
new data items.  This fix this was inspired by the online checksums
patch which adds data to this record,  but this change has value on
its own.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/c92b5d8b-bc03-47bc-b209-2e4a719eee32@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/097ab69d17f74e3c45b8c092d29df74f59bb5421

Modified Files
--------------
src/backend/access/rmgrdesc/xlogdesc.c | 6 +++---
src/backend/access/transam/xlog.c      | 8 +++++++-
src/backend/postmaster/walsummarizer.c | 5 ++++-
src/include/access/xlog_internal.h     | 6 ++++++
src/tools/pgindent/typedefs.list       | 1 +
5 files changed, 21 insertions(+), 5 deletions(-)