Traits reform
Correct pointing cursor to exact item
Load only key while checking for key validity
Use MDB_SET_KEY to ensure key validity

The thing is that if MDB_SET just points to key, but key value should be
kept in cursor as it might not outlive cursor. MDB_SET_KEY copies both
key and data back into provided buffers so after setting cursor we have
a valid internal pointer to key.
New way to create databases
Update to nightly
Dropped MdbWrapper
Keyrange from key and to key
Fixed lifetime shadowing warnings
Small iterator cleanup
Removed commented code