Re: EXPLAIN: showing ReadStream / prefetch stats

Hi,

I've pushed the first two parts earlier today. Here's a v13 with the
remaining pieces rebased on top of that, with updated commit messages
and some additional fixes.

1) There was a bogus else-if in explain.c that would make it impossible
to enable "wal" and "io" at the same time.

2) Replaced one of the "typedef IOStats" with just forward declaration,
to prevent possible conflicting typedefs in the future (even if that
wasn't an issue now).

3) Used (es_instrument & INSTRUMENT_IO) more consistently. A couple
places in the executor still checked just es_instrument, and so would
collect stats even if not needed. Be consistent.

4) Various comment additions / improvements.

I intend to push this later today, after giving it one more round of
review etc.

regards

--
Tomas Vondra