Distinguish datacheckums worker invocations more reliably

Distinguish datacheckums worker invocations more reliably

In some corner cases, a new datachecksums worker could be launched
while an old one was still running.  If you're really unlucky, the old
worker could set the worker_result in shared memory and mislead the
launcher to think that a newer worker invocation completed
successfully, even though it failed for some reason.  That's highly
unlikely to happen in practice as it requires several race conditions
with workers and launchers starting, failing and succeeding and at the
right moments.  Nevertheless, better to tighten it up.

To distinguish different worker invocations, assign a unique
'worker_invocation' number every time a new worker is launched.  In
the worker, check that the invocation number matches before setting
the worker result.  This ensures that the result always belongs to the
latest invocation.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/b283fbb9-298e-4953-9120-eefaf24fae20@iki.fi