Backport GetWALInsertionTimeLineIfSet()

Backport GetWALInsertionTimeLineIfSet()

This routine is able to return the WAL insertion timeline.  Contrary to
GetWALInsertionTimeLine(), this can be called before recovery is marked
as done in shared memory, just after InsertTimeLineID has been set.  So
it can offer more flexibility in terms of timeline detection without
depending on RecoveryInProgress().

This routine will be used in a follow-up fix that will be applied down
to v16, and it has been introduced in 53b327f83ea2 (v17~).

Discussion: https://postgr.es/m/7daef094-abf3-4672-bc23-3df4763b16a3@gmail.com
Backpatch-through: 16