[WriteTransaction] Make WriteTransaction a subclass of Transaction

This patch refactors Transaction / WriteTransaction code. It solves following issues:

• Checks whether one is allowed to perform write operations on DB are performed at compile time instead of having bug-prone asserts. Now one has to use Transaction for read-only access and WriteTransaction for read-write access.
• All modify operations are moved to WriteTransaction. Previously Transaction contained some code which actually modified the DB (setPhaseOne, removePhaseOne and addFailed), which contradicted the original idea of separation read-only and read-write access by having two classes.
• It also allows to simplify the Transaction code and drop shallow write-related proxy-methods that forwarded those calls to WriteTransaction.