Override for block hashing for block 202612

Since we need to fix tree_hash, but doing so would invalidate the block
id for block 202612, this fix should check to see if we're trying to get
the block id for 202612 (if its blob hash matches) and return the "old"
block id, for backwards compatibility.
hex_to_pod needs the destination as an arg, as opposed to it returning the pod
added double-check for 202612 block id

if a new block has the same block id as 202612 but the wrong blobdata,
this will tell the caller that the block id is actually null_hash rather
than the 202612 block id.
override for get_block_longhash for block 202612
Fix tree-hash cnt n^2. Asserts, comment. Squash2
checkpoints on restore; currently fails on 212 checkpoint
bug fix to checkpoint-on-restore; still fails on 612
fix checkpoints