[CHIA-3654] previous transaction block height in BlockTools

[arvidn]

Purpose:

pass the correct previous_transaction_block_height into validate_pospace_and_get_required_iters() in BlockTools.

The main change is in load_block_list() in BlockTools. We call this function when block tools is asked to generate blocks on top of an existing chain. Rather than always setting prev_transaction_b_height to 0, we hoist it out of the loop and update it as we encounter transaction blocks.

The other change is the tracking of previous transaction blocks in the main loop in get_consecutive_blocks(), only update prev_tx_height when we actually produce a transaction block.

[wjblanke]

aok