Merge branch 'master' into t

* master:
  go/zodb/btree: Change V<op> family to also provide visited node key coverage on visit callback
  go/zodb/btree: Add KeyRange type
  go/zodb/btree: Introduce constants for min/max key value
  go/zodb/btree: tests: Don't forget to close storage
  go/zodb/btree: Cosmetics