Unify DatabaseBroker.reclaim

Both AccountBroker and ContainerBroker install their own reclaim()
methods, which are the same, modulo the table and column names.
Also, the parent reclaim() method is unused, except as a tramptoline
to test _reclaim().

This patch gets rid of the unused code by moving it into the tests,
and unifies the reclaim() method in DatabaseBroker. We use the
same class parameters that are already in use elsewhere.

The unused code was found when documenting the DB broker API for
the LFS. The unification fell out of reviews by Clay and Peter.

Change-Id: Ic5e38a9e39dafe8e0bc062818151edece384f3b5
Merge "Unify DatabaseBroker.reclaim"