De-dupe receipt IPLDs

[i-norden]

In an effort to minimize duplication of data in our IPLD database, we want to

1. No longer publish Receipt IPLDs to the public.blocks table
2. Replace the mh_key and cid fields in the receipt_cids table with a multihash and cid that instead point to the ReceiptTrie leaf node IPLD that contains, as the value stored in the leaf, the receipt we were previously publishing and referencing directly
3. We need to make it clear that the IPLD we reference is a leaf node and not a standalone receipt, so we should consider renaming the rows to leaf_mh_key and leaf_cid

[i-norden]

Jumped the gun, waiting on this to be merged: cerc-io/ipld-eth-server#103