Irreversible mode improvements

[heifner]

• Return apply_blocks result in irreversible mode so incomplete apply is handled correctly.
• Handle interrupt in irreversible mode without logging an error.

Test failed because irreversible mode nodeos would not transition to Savanna. It would not transition because the log_irreversible was incomplete but nothing called apply_blocks again to continue the process.

Resolves #1082

[greg7mdp]

I think that this would have been equivalent to leaving the check in should_process, setting result to incomplete before entering the loop, not updating it within the loop, and setting it to complete after the loop if the iterator is at end().

[heifner]

I think that this would have been equivalent to leaving the check in should_process, setting result to incomplete before entering the loop, not updating it within the loop, and setting it to complete after the loop if the iterator is at end().

If you think that is clearer, I can change it.

[greg7mdp]

If you think that is clearer, I can change it.

Up to you what you think is best since you are the one maintaining this code generally. I'm fine either way.