Deal with new pricing structure

[rvagg]

These are reported on master while using --auto-fund:

InsufficientFundsForMinimumRate(address payer, uint256 minimumRequired, uint256 available)
                               (0xAf992Fbc0c22BC941A232c63dc1b0c0cD572D145, 60000000000000000, 216443055441240)

We are dong base price * size, but we need to do min(floor price, base price * size) there.

This depends on FilOzone/synapse-sdk#360 being fixed so we have the right info.

[SgtPooki]

I believe this is fixed with #218, but we probably need a tracking item to use more pricing verification from synapse once FilOzone/synapse-sdk#375 and FilOzone/synapse-sdk#360 are resolved.