R1:c30cb7669e37

Simplify transaction handling and restore read/write locks

Summary:
- We used to have connection-level caching, so we needed getTransactionKey() to make sure there was one transaction state per real connection. We now cache in Lisk and each Connection object is guaranteed to represent a real, unique connection, so we can make this a non-static.
- I kept the classes separate because it was a little easier, but maybe we should merge them?
- Also track/implement read/write locking.
- (The advantage…