verify_state_blocks() with a max is O(n)

[rkeene]

verify_state_blocks() with a max parameter (added in #1450) is O(n)

with 1000000 items in the state_blocks deque it takes much more time to do the swap than with 10000 items, given the same "max" parameter

[rkeene]

Relevant section: https://github.com/nanocurrency/raiblocks/blob/49b047d0c78c440cdd283cbdfb3f97bd09731446/rai/node/node.cpp#L1311-L1315

[cryptocode]

Here are my notes:

auto position = std::distance(state_blocks.begin(), state_blocks.end () - keep_size); => 2048
so swap_ranges gets to iterate the whole shebang minus max_count
that said, resize is taking even more time in my measurements depending on deque size

[SergiySW]

Closing with #1571