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.
WIP: debugging key issues
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