Update *.header_cids.node_id to *.header_cids.node_ids

[i-norden]

Replace the node_id field in the btc and eth header_cids table (e.g. https://github.com/vulcanize/ipfs-blockchain-watcher/blob/master/db/migrations/00004_create_eth_header_cids_table.sql#L10) with a node_ids integer array field. When a unique node source inserts a duplicate header, they add their id to this array. This serves as a means to track the "breadth" of data as a measure of validity.