Re: pg_*_advice: tsv load failure, etc.

Hello

I/Claude noticed one more issue with pg_stash_advice:
if the worker fails to write pg_stash_advice.tsv, the changes it was
trying to save are treated as saved by the next worker.

Any ERROR in pgsa_write_to_disk() makes the worker exit, and the
postmaster restarts it later. stashes_ready is already set, so the new
worker doesn't load the file, and it initializes last_change_count
from the current change_count.

From that point the worker considers everything in memory clean, the
file is only rewritten after some later change or at a clean shutdown.
An immediate shutdown or a crash in between reloads the old file, and
the changes are gone, even though the worker was running for much
longer than persist_interval.

The attached patch starts last_change_count from 0 in a worker that
didn't load the file itself. This writes at the first opportunity
whenever unsaved changes exist. It can cause one redundant rewrite
after a restart, which seems harmless. I also moved the change_count
read in the loading branch before setting stashes_ready, otherwise a
backend change between the two could be counted as loaded.

A TAP test is included for reproduction and as a permanent test case
if we want it.