Spend down the block budget limit by x% every block

[jferrant]

@hstove did substantial work on this PR as well so will need review from two others.
Closes #5433

[jferrant]

@obycode also recommends making this a configurable, but also to make it a soft limit and to apply it after all other checks. Will move it to try_mine_tx_with_len in NakamotoBlockBuilder.

[jferrant]

@obycode also recommends making this a configurable, but also to make it a soft limit and to apply it after all other checks. Will move it to try_mine_tx_with_len in NakamotoBlockBuilder.

I tried to make this a soft limit but it seemed to require some pretty ugly messing around with the way we call try_mine_tx_with_len in select_and_apply_transactions and how we use that result before exiting our loop. Still working on fixing this. EDIT: think I got it working. Gotta fix CI and Tests.

[obycode]

We could avoid this check altogether if a percentage of 100 in the config becomes a None instead of Some(100).

[jferrant]

True, but i kept this additional check in case someone wrote code directly instead of using the raw config file. i.e. we shouldn't attempt to increase the amount of block space nor set it to zero no matter what, I had assumed. But prob better to not fail silently anyway.. maybe convert this to an assert here in case there is a poorly written test? I.e. assert!((1..100).contains(percentage), "BUG: Invalid block budget percentage {percentage}") ?