- Load does not return "next serial"
- Load always returns LoadError wrapping error cause
- Load: clarified & documented semantic for "there-is-no-data" results
- ErrOidMissing -> NoObjectError (just rename)
- ErrXidMissing -> NoDataError   (rename + adjust information inside)

  inside: xid -> oid + deletedAt

  deletedAt is needed if storages are stacked (e.g. in DemoStorage) in case
  there is no data for oid/xid in "changes" to find out whether in "changes" oid
  was created after searched xid.At (and then look into "base"), or it was
  already deleted in "changes" and then we must not look into "base".

  see: kirr/ZODB@a762e2f8

- URL IStorage -> IStorageDriver (so that raw driver can properly return
  LoadError from its load)