[Bugfix + Feature request] transaction.gasUsed is not gasUsed

[Amxx]

Today, when handling an event, the transaction object contains gas-related entries:

• event.transaction.gasPrice
• event.transaction.gasUsed

However, event.transaction.gasUsed does not contain the gas used but the gas limit (which is part of the transaction). Gas used (and cumulative gas used) are part of the transaction receipt.

Steps that should be taken:

• Rename the current gasUsed into gasLimit
• Add gasUsed and/or cumulativeGasUsed from the transaction receipt in the event.transaction object

[leoyvens]

We could put a receipt field in the transaction, without all the logs if that's not necessary. But while we do currently pull in all receipts, that's a wasteful thing we do that I hope to refactor away as part of graphprotocol/graph-node#902. So a receipt field should be lazily loaded and fetch the receipt on-the-fly, though we can cache it.