Timing issue in pg_basebackup

Hey!

I recently came across this issue when running pg_basebackup from 2
nodes at the same time through ansible. One of the nodes sometimes
reported: "requested WAL segment ... has already been removed". Tested
on v16, v17 and v18. The workaround was running them in sequence or
pause between runs.

It looks like there's a timing issue here:
1. Backup A starts and runs a checkpoint - needs WAL from segment 2.
2. Backup A hasn't reserved that WAL yet.
3. Backup B starts and runs a checkpoint (or anyone runs a checkpoint) -
deletes segment 2.
4. Backup A asks for segment 2, which is gone, and fails.

I've included a patch that moves the replication slot handing to run
before the backup part as a separate function.

Some possible caveats already:
* Since moving the slot creation, we might run in to trouble if for
example, --checkpoint=spread is used since the slot is open for longer.
* If any error arises in the backup part, now there will be a lingering
slot when previously no slot was created.

*** DISCLAIMER ***
I've used an LLM to help me write the test - I'm rubbish at Perl.

Love to get some feedback.

Regards,
Jesper St John