aio: Fix possible state confusions due to interrupt processing

aio: Fix possible state confusions due to interrupt processing

elog()/ereport() process interrupts, iff the log message is < ERROR and the
log message will be emitted. aio's debug messages are emitted via ereport(),
but in some places the code is not ready for interrupts to be processed.

Fix the issue using a few different methods:

1) handle interrupts arriving concurrently - in some places it's easy to
   detect that by fetching the handle's generation a bit earlier
2) Check if interrupts made the work needing to be done obsolete
3) Disallow interrupts, as there's no sane way to make interrupt processing
   safe

To prevent some similar issues from being re-introduced, assert that
interrupts are held in pgaio_io_update_state().

This commit also fixes the contents of a debug message I added in 039bfc457e4.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/mvpm7ga3dfgz7bvum22hmuz26cariylmcppb3irayftc7bwk3r@l7gb6gr7azhc