Factory re-implements
Jdbi.onDemand(Class) in order to create proxy, using transactional handler
(jdbi on-demand objects supposed to open-close connection on each call).
Proxy is not created immediately because during proxy creation config is created for each method and so
if some global row mapper will be registered after this moment, method config will not know about it.
Provider is created just before injector creation and all mappers are registered just after injector creation,
so without laziness nothing would work as planned.