Implement new config option 'isolation_level' in rdbms section. This is
only relevant for postgres and mysql (sqlite does locking and implements
a very high isolation level by default :-)

Thanks John for the hint that mysql also implements this.

Add a test for Postgres that reproduces the observed history behaviour
with isolation level 'read committed' and throws a serialization error
with level 'repeatable read'.

The isolation level 'read committed' is the new default (which was the
case for postgres before but mysql by default seems to have repeatable
read by default)