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

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 <unlimitedhikari@gmail.com>
Reported-by: ChangAo Chen <cca5507@qq.com>
Author: Andrey Rudometov <unlimitedhikari@gmail.com>
Author: ChangAo Chen <cca5507@qq.com>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: ChangAo Chen <cca5507@qq.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Discussion: https://postgr.es/m/CAF6JsWiO=i24qYitWe6ns1sXqcL86rYxdyU+pNYk-WueKPSySg@mail.gmail.com
Discussion: https://postgr.es/m/tencent_E00A056B3953EE6440F0F40F80EC30427D09@qq.com
Backpatch-through: 18