add(rpc): Add block times to verbose output of getblock RPC method

[arya2]

Motivation

This field is provided in zcashd and was asked about on Discord.

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Solution

• Adds BlockHeader state requests/responses (there were already FindBlockHeaders requests/responses, this one is simpler)
• Requests the block header from the state service in the getblock() RPC method
• Adds the timestamp (seconds since the unix epoch) as a time field to the verbose return value from the getblock() RPC

Testing

Update the existing snapshot and vectors tests to include block time.

Review

Anyone can review.

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Follow Up Work

• Adding other fields from the block header to the getblock RPC's verbose output

[arya2]

We may want to get the full block data from the state service for verbosity = 1 instead, because it'll be needed for returning the serialized block sizes and auth data roots (see https://github.com/zcash/zcash/blob/master/src/rpc/blockchain.cpp#L225).

Or we may not need to match zcashd's output exactly.

Update:
There's a note about this RPC being used for lightwalletd's initial sync, so, for now it needs to load all its fields very efficiently.

If we do want to match zcashd's output, we could:

• Move or duplicate the debug_like_zcashd config from the mining section to the rpc section and only read the entire block when it's enabled, and/or
• Add an optional argument for callers to provide a list of fields to be included in the return value.

[mpguerra]

It seems like the scope for this PR is possibly expanding. Should we create an issue for all of the things we may want to do in order to support lightwalletd and potentially also match zcashd behaviour?