[Consensus][small] Pacemaker timeout calculations ignore nil blocks
[Consensus][small] Pacemaker timeout calculations ignore nil blocks

Summary:
We recently introduced NIL blocks, which can now commit even if the execution of a block takes more time than the round duration.
Unfortunately it might lead to a situation in which the round duration is consistently smaller than the execution time but it doesn't grow because of the NIL blocks.
This diff does not notify the Pacemaker about the commits of the NIL blocks, this way the highest commit round kept by Pacemaker is the highest commit round of the non NIL blocks.

Test: current unit test coverage

Tasks: ref #371

libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core