fix wrong param in prevalidate

[almogdepaz]

Purpose:

fix a bug where we pass the wrong blockchain data structure into get_next_sub_slot_iters_and_difficulty

Current Behavior:

we pass self.blockchain instead of the AugmentedBlockchain param passed into get_next_sub_slot_iters_and_difficulty
this means that we rely on having the blocks in the cache, which now happens when we call add blocks but is redundant since we also use AugmentedBlockchain to stage the new blocks before we switch to the new fork

New Behavior:

pass the correct parameter

Testing Notes:

[arvidn]

we really should have a test that exercise this. I'm a bit surprised the deep reorg tests don't actually. We have a case where we switch from lighter blocks to heavier ones, which I believe should affect the difficulty

[almogdepaz]

we really should have a test that exercise this. I'm a bit surprised the deep reorg tests don't actually. We have a case where we switch from lighter blocks to heavier ones, which I believe should affect the difficulty

we added the AugmentedBlockchain but didn't all the code paths that support the old cache flow, so we have a safety net that allows this to work so i am not sure we can write a test that throws here but we do keep redundant data and this can lead to bugs if someone changes the code expecting it to work entirely with AugmentedBlockchain