Added tests for executor and some improvements:
1. Add a dependency map during execution. This will prevent aborted tasks from being sent for execution immedaitely after failure.
2. Change the key of MVHashMap from string to a byte array. This will reduce time to convert byte slices to strings.
3. Use sync.Map to reduce the time spent in global mutex.
4. Skip applying intermediate states.
5. Estimate dependency when an execution fails without dependency information.
6. Divide execution task queue into two separate queues. One for relatively certain transactions, and the other for speculative future transactions.
7. Setting dependencies of Txs coming from the same sender before starting parallel execution.
8. Process results in their semantic order (transaction index) instead of the order when they arrive. Replace result channel with a priority queue.
In `touch` operation, only `touched` filed has been changed. Therefore
in the related undo function, only `touched` field should be reverted.
In addition, whether remove this obj from dirty map should depend on
prevDirty flag.
This commit replaces the deep-copy based state revert mechanism with a
linear complexity journal. This commit also hides several internal
StateDB methods to limit the number of ways in which calling code can
use the journal incorrectly.
As usual consultation and bug fixes to the initial implementation were
provided by @karalabe, @obscuren and @Arachnid. Thank you!
