pgsql: Add assertions checking for the startup process in WAL replay ro

Add assertions checking for the startup process in WAL replay routines

These assertions may prove to become useful to make sure that no process
other than the startup process calls the routines where these checks are
added, as we expect that these do not interfere with a WAL receiver
switched to a "stopping" state by a startup process.

The assumption that only the startup process can use this code has
existed for many years, without a check enforcing it.

Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/aQmGeVLYl51y1m_0@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2fc31079624e16d6b1a8508958dbf81e8edce057

Modified Files
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src/backend/access/transam/xlog.c         | 2 ++
src/backend/access/transam/xlogrecovery.c | 4 ++++
2 files changed, 6 insertions(+)