Batch signature verification for poc receipts and witnesses

[vihu]

Problem

Both poc witness and poc receipt signature verification process times can be increased significantly if we do it in batches.

Solution

• Add a boolean flag to is_valid function for poc_receipt and poc_witness
• Add verify_signatures function for poc_receipt and poc_witness
• Invoke batch verification for poc_receipt and poc_witness in poc_receipts_txn

TODO:

• [ ] Resyncing
• [x] Remove debugging
• [x] Does this need a chain variable, I suppose we'll find out with resync
• [ ] Check that the new error invalid_receipt_signatures does not break past behavior