Handle null blocks

[dboreham]

• Find nearest non-null "to" block when scanning block range triggers

• Make ancestor_block short-circuit at a root ancestor hash, rather just checking relative block
  offset

• use nearest_block_hash_to_number in is_on_main_chain

• Substitute nearest block in BlockChain::block_pointer_from_number

• Patch load_block_ptrs_rpc

• Add vulcanize copyright notices