Fix background worker not restarting after crash-and-restart cycle.

Previously, if a background worker crashed (e.g., due to a SIGKILL) and
the server restarted due to restart_after_crash being enabled,
the worker was not restarted as expected. Background workers without
the never-restart flag should automatically restart in this case.

This issue was introduced in commit 28a520c0b77, which failed to reset
the rw_pid field in the RegisteredBgWorker struct for the crashed worker.

This commit fixes the problem by resetting rw_pid for all eligible
background workers during the crash-and-restart cycle.

Back-patched to v18, where the bug was introduced.

Bug fix patches were proposed by Andrey Rudometov and ChangAo Chen,
but this commit uses a different approach.

Reported-by: Andrey Rudometov <[email protected]>
Reported-by: ChangAo Chen <[email protected]>
Author: Andrey Rudometov <[email protected]>
Author: ChangAo Chen <[email protected]>
Co-authored-by: Fujii Masao <[email protected]>
Reviewed-by: ChangAo Chen <[email protected]>
Reviewed-by: Shveta Malik <[email protected]>
Discussion: 
https://postgr.es/m/CAF6JsWiO=i24qyitwe6ns1sxqcl86ryxdyu+pnyk-wuekps...@mail.gmail.com
Discussion: 
https://postgr.es/m/[email protected]
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/75f633f54aaae4d20ea0ade9a953afa73e3a66e8

Modified Files
--------------
src/backend/postmaster/bgworker.c   | 1 +
src/backend/postmaster/postmaster.c | 7 +++++++
2 files changed, 8 insertions(+)

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