Support block payloads that are too small to hold the entire tx table

[ggutoski]

If the tx table declares a large length (eg. 4 billion) then the block payload might end before the tx table is complete. In that case every tx in the block has zero length, which is fine. But if anyone calls QueryablePayload::iter() then they'll get an iterator over 4 billion empty txs, which is bad.

The solution is to define the tx table length as min(declared length, number of words that can fit into the payload).