Refactor lock and unlock logic

At the moment, there is not a clear separation between the UnlockDatabase, UnlockedDatabase and DatabaseManager and the code to lock and unlock a database is split between these 3 classes.

The main idea of this MR is to move all this logic in the DatabaseManager class. This is achieved by adding a locked property to it. Then, UnlockDatabase and UnlockedDatabase listen to the changes of this property to update accordingly.