This change makes the StateDB track the state key value diff of a block transition.
We already tracked current account and storage values for the purpose of updating
the state snapshot. With this PR, we now also track the original (pre-transition) values
of accounts and storage slots.
- Move the existing tests from memorydb into a generalized testsuite
that can be run by any ethdb backend implementation.
- Add several more test cases to clarify some non-obvious nuances when
implementing a custom ethdb backend, such as the behaviour of
NewIteratorWithPrefix vs NewIteratorWithStart.
- Add leveldb to the testsuite using in-memory storage for fast
execution.