Fix estimateGas being not accurate

[yann300]

using runTxInVm for the gas estimation isn't accurate because it doesn't take in account block.number, block.timestamp, etc...
to test, run:

contract test {
 uint number;
 function set() public {
   number = block.timestamp;
 }
}

The fix make the estimateGas to also use runBlockinVm, and implement a way to rollback the state proposed by ethereumjs team.

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