Do contextual validation on mempool transactions using the state best tip

[teor2345]

Motivation

The getblocktemplate RPC collects the transactions in the mempool into a block. That block must be contextually valid based on the best tip in the state.

We'll want a new state request for this.

Design

Contextual validation: https://github.com/ZcashFoundation/zebra/blob/afb8b3d4775d89b3f9223447341bf6ce152f5c3a/book/src/dev/rfcs/0002-parallel-verification.md

State design details: https://github.com/ZcashFoundation/zebra/blob/afb8b3d4775d89b3f9223447341bf6ce152f5c3a/book/src/dev/rfcs/0005-state-updates.md

[teor2345]

@mpguerra this is part of the getblocktemplate work I was talking about on Slack.

[mpguerra]

Hey team! Please add your planning poker estimate with Zenhub @arya2 @conradoplg @oxarbitrage @teor2345 @upbqdn