Make scheduling a pre-commit smarter

[th7nder]

          I didn't thought about the start_block, that's a good point.

I only thought about accumulating deals until full and time-based like "every hour, unsealed sectors get sealed and move on".

The issue with the block based dependency is that we need to know how early we can schedule blocks for, i.e. — how long it takes us to seal and pre commit sectors so that we avoid accepting deals that we won't able to close on time

Originally posted by @jmg-duarte in #483 (comment)