[5.0] Normalize speculative & producer block intervals

[heifner]

#1481 changed speculative block creation so that speculative blocks would always have a current block time. However, it didn't correctly handle the case where the configured producer was the scheduled producer but could not produce (e.g. incorrect configured private key). This PR simplifies the implementation to just use the producer schedule as configured by produce-block-offset-ms regardless if the block is speculative or a producer block. This normalizes block production time for speculative and producer blocks. I think this normalization is better than maintaining different block intervals for speculative vs producer nodes.

Resolves #1837

[linh2931]

Is it possible to add a test where the scheduled producer could not produce?

[heifner]

Is it possible to add a test where the scheduled producer could not produce?

I think that would be under #1804

There are tests in test_block_timing_util.cpp that test the timings, so I feel the functionality is well tested; just not the actual usage here.

[linh2931]

I think that would be under #1804

There are tests in test_block_timing_util.cpp that test the timings, so I feel the functionality is well tested; just not the actual usage here.

I meant the situation reported by #1837. No more lots of "not producing" messages after this fix. But if it is too much effort, there is no need to do that, as the fix makes things simpler. I am approving the PR.