Refactor BlockTools

[arvidn]

Purpose:

Simplify BlockTools to make it easier and more robust to additions and changes.

This PR is best reviewed one commit at a time.

One realization (that's mentioned in a new comment in this PR) is that we don't know whether a block will end up a transaction block or not up-front. We always prepare the transactions and pass into get_full_block_and_block_record(), only afterwards do we know whether the transactions were included or not.

The include_transactions feature (which includes some number of transactions in each TX block) maintains a list of available_coins that it spends some of, and add the coins created by those spends. These spends used to rely on another feature transaction_data (which can also be passed in by the caller), in order to "wait" until we get a transaction block to make the next set of transactions.

This patch removes this coupling and instead explicitly updates available_coins every time we get a transaction block. This makes the transactions not be created identically to the current chains and causes validation failures. That's why this needs an updated test-cache version of the blockchains. (available here)

Current Behavior:

• there is a lot of code that's duplicated in get_consecutive_blocks().
• there are unintuitive (and difficult to follow) dependencies between loop iterations when generating multiple blocks
  • include_transactions feature generate dummy transactions by setting transaction_data
  • transaction_data_included is set to true when transaction_data is included into a block that turned out to be a transaction block. We then set transaction_data to None to not include it again.
  • the start_height parameter was unused
• when generating transactions, we track available_coins and we remove the coins immediately when we create the bundle. This means we must keep the bundle around until the generated blocks turns out to be a transaction block (hence the dependency on transaction_data.

New Behavior:

• there is slightly less code duplicated (some code was factored out into setup_new_gen())
• there code to generate blocks is more insulated and has fewer unexpected dependencies. For example
  • the transaction_data feature is now independent from include_transactions feature.
  • transaction_data_included was removed. transaction_data is set to None directly
  • the start_height parameter was removed
• when generating transactions, we track available_coins and we remove coins we spent if the block turned into a transaction block

Testing Notes:

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