fix(rpc): Await state service requests in getblock method in parallel

[arya2]

Motivation

This is cleanup in the getblock method ahead of adding any additional service calls, adding other fields to the getblock response that are provided in zcashd would likely require another state service call to request the block header, and we may also want to look up the height if the method was called with a block hash.

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Complex Code or Requirements

This could change the order in which the state service receives and responds to these requests, which should be okay.

Solution

• Calls state.oneshot() instead of .ready().and_then(svc.call()) for getting the transactions ids, sapling/orchard trees, and block depth
• Awaits the futures in a FuturesOrdered

Testing

Existing tests should cover this change.

Review

Anyone can review.

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Follow Up Work

• Look up block heights and headers in the state service
• Add fields in zcashd to GetBlock::Object verbose response of the getblock method
• We may also want to:
  • Use a join!() instead of a FuturesOrdered (not done here due to rustfmt issues)
  • Move the error conversion to a trait method + From impl to avoid duplicate code

[oxarbitrage]

Looks a lot better. Thank you.