Check if round has progressed before calling monitorProgress

[samliok]

1. we are building a block
2. while we build a block we get an empty notarization
   a .we increase the round,
   b. the block builder gets cancelled
   c. we call monitorProgress on the new round when we enter startRound
3. we call the monitorProgress in the buildBlock task after. this would cancel the current monitorProgress and monitors a stale round

therefore we should check if the round has increased while building a block and not call monitorProgress() if so

[yacovm]

#267 has a test :)