Update to only fetch transactions for blocks when necessary.

[akhila-raju]

Implements #49.

[raulk]

@akhila-raju a few comments:

• Where is the new logic being tested? Ideally we'd be having tests that verify that web3.eth.getBlock() was called with the second argument set to true/false depending on the cases. I think you can use jest.spyOn() for this.
• package-lock.json is not a file we want in the repo. Perhaps you're using npm/npx instead of yarn?

[CLAassistant]

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[raulk]

@akhila-raju pushed a few commits here to:

• Consider only immediate children to avoid false positives when transactions are requested for nested blocks, not the block at hand (e.g. parent, ommers).
• Remove dependency on graphql-list-fields.
• Reorganize related tests into a dedicated suite.
• Field whitelist is now in a static field.

Further refactoring is required in the model, as it's questionable for this logic to be in the EthqlQuery root query, it should be in the EthqlBlock class so it can be reused across other fields that yield a Block (parent, ommers, and others down the line).