node needs a minimum time for block building

[just-mitch]

Presently, if the TX pool is empty and then it comes my turn to propose, I will still attempt to build and publish the block even though I only have e.g. 3 seconds left in my slot.

We need an environment variable that specifies the minimum amount of time node thinks it needs in order to build and publish its block in time.

[just-mitch]

Fixed in #9767