Investigate checking the square sizes that are signed over before estimating square size

[evan-forbes]

Currently the block producer assumes that the txs being passed to it in PrepareProposal have signatures for whatever square size that it picks. This is particularly problematic when there are only a few transaction in the mempool that are only signed over large square sizes. This results in those txs not being included until a larger block is created, which will not occur until their are enough transactions passed to PrepareProposal.

The easiest way to prevent this is to sign over all relevant block sizes by default #239, but we might want to consider also checking the txs that are passed to PrepareProposal before we estimate square size. If we do that, then we can check for this exact situation. We would end up producing a less efficient block, as there would be a lot of tail padding shares added, but at least the users txs wouldn't get stuck in the mempool until a larger block is created.

[evan-forbes]

we can close this since by default we are signing over all square sizes and checking for this in the mempool #666