Better timestamps on simulated chains

[arvidn]

This PR is best reviewed one commit at a time.

Purpose:

This updates the BlockTools logic to generate more authentic simulated chains, and also lets us generate the long chains faster.

The highlights are:

• The (simulated) timestamps on blocks are proportional to the number of iterations between the blocks
• The logic to support tracking of sub-second simulated time is simplified to just use float, instead of integer seconds + residual

Current Behavior:

The timestamp between two transaction blocks is always 18 seconds, regardless of there being non-tx blocks in between
The timestamp between blocks is irrespective of the number of actual VDF iterations between the blocks. This effectively simulates a timelord that speeds up over time, making it increasingly expensive to generate the chain.

New Behavior:

The timestamp between two transaction blocks is (by default) 18.75 seconds for each block (including non-TX blocks). This time delta is scaled by the number of iterations.
The simulated chain does not simulate a timelord that speeds up over time. The cost of generating the chain is linear to the number of blocks.

Testing Notes:

I generated the chains, including the 10000 block one with the debug output to verify.

[emlowe]

Do we need new plots checked into the plot cache repo?

[arvidn]

Do we need new plots checked into the plot cache repo?

Yes, see: Chia-Network/test-cache#7