r15795: Try to use the async code by default

r15795: Try to use the async code by default
It passess all my tests, but I still need to work on a lot of stuff.
Shouldn't impact anybody else work, so I want to commit now and see what happens

Will work to remove the old code from modules and backends soon, and make some
more restyling in ldb internals.

So, if there is something you don't like in this desgin please speak now.

Simo.
(This used to be commit 8b2a563e716a789ea77cbfbf2f372724de5361ce)