Simplify BlockExecutor Logic

[aeidelman]

nitially, BlockExecutor was a simple object that runs the transactions in a block, and in other method, apply the results (root store, etc...) to fill a block. Now, the execution of the transactions is polluted by the use of flags. And it is used with different flags from different core code (miner server vs blockchain). And one flag is used to skip some "not well formed" transactions in old tests. 

We should refactor this class to have a clearer use, and improve to have test with "real and accepted and well-formed" transactions