Re: let's kill AtSubStart_Notify

Generally, a subsystem can avoid needing a callback at subtransaction
start (or transaction start) by detecting new levels of
subtransactions at time of use.

A typical practice is to maintain a
stack which has entries only for those transaction nesting levels
where the functionality was used. The attached patch implements this
method for async.c.

I was a little surprised to find that it makes a
pretty noticeable performance difference when starting and ending
trivial subtransactions.  I used this test case:

\timing
do $$begin for i in 1 .. 10000000 loop begin null; exception when
others then null; end; end loop; end;$$;