RFC: Use Metadata Cache for all operation

Move metadata cache from DNSSECKeeper to UeberBackend.
DNSSECKeeper::clearCaches, clearAllCaches forwards to UeberBackend.

Important changes:
* UeberBackend got yet another cache.
* UeberBackend::setDomainMetadata will now clear the metadata cache.
* reload, rediscover will now clear the metadata cache.
* In DNSSECKeeper the ops counter was shared across both caches,
  and across more operations. I have changed the required number of
  ops (before triggering cleanup) in UeberBackend to a tenth of the
  DNSSECKeeper value, but this is just a wild guess.

Note that this is mostly cut and paste coding, and little to no
testing has been done.