Previously we were just checking it exists in the DB. This is incorrect
because the last_justified_block_hash _must_ be in the chain referenced
by the block.
I.e., it's not OK for a block to reference a justified block in another
chain.
This adds block and attestation validation code that was written
previously. There were many non-validation specific changes made whilst
building these functions (e.g., db, hashing, etc) -- these changes have
already been merged into master and this branch has been created just to
make it easy to review this code.
