proof-of-space v2 support

[arvidn]

Purpose:

This PR adds support for v2 plots to chia-blockchain, activating at the hard fork height (which is still set to a place-holder).

These changes are best reviewed one commit at a time.

The main updates can be summarized as:

• add support for creating v2 plots in BlockTools (the core of the simulator)
• add two new ConsensusMode enum values to the test fixture. This will make many of our tests run two more times covering the v1 phase-out and v2-only chains.
• fixing tests to work with v2 plots. The main issue is that v2 plots don't support a pool key, so pool rewards can't be redirected. Pool rewards can only be paid to a puzzle embedded in the plot itself. Many tests redirect both the farmer reward and the pool reward, so the change is to rely only on the farmer reward, at least after the hard fork activation.
• disabling tests that depend on the pool reward redirect, to be fixed later. These are also marked with the todo_v2_plots keyword, to make them easy to find and identify.

Current Behavior:

• We don't have any tests covering the 3.0 hard fork

New Behavior:

• We run most existing tests on the 3.0 hard fork as well