Migrating task model and backend to use typed_sql.

[isoos]

• This reverts commit 3796fab.

[isoos]

Note: this is re-applying the original rewrite + layering several commits on top of it that address the re-entrant transactions, the query + transaction on the same connection pattern and simplify further parts, e.g. how the dependency timestamp gets updated and read periodically.

[sigurdm]

I guess you disconnect the stacktraces here. Not sure if it is important.

[isoos]

The previous version did disconnect them. The new code preserves the full stacktrace, puts them into a concurrent pool, and then awaits them. I just don't await the individual future that the pool returns. Previously we have used:

  final f = pool.schedue...
  futures.add(f);
  
  await Futures.wait(futures);

But I think it is no longer necessary with this block + awaiting the pool.close();