Fix re-initialization of LWLock-related shared memory.

Fix re-initialization of LWLock-related shared memory.

When shared memory is re-initialized after a crash, the named
LWLock tranche request array that was copied to shared memory will
no longer be accessible.  To fix, save the pointer to the original
array in postmaster's local memory, and switch to it when
re-initializing the LWLock-related shared memory.

Oversight in commit ed1aad15e0.  Per buildfarm member batta.

Reported-by: Michael Paquier <michael@paquier.xyz>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aMoejB3iTWy1SxfF%40paquier.xyz
Discussion: https://postgr.es/m/f8ca018f-3479-49f6-a92c-e31db9f849d7%40gmail.com