Merge #13452: rpc: have verifytxoutproof check the number of txns in proof structure

d280617bf569f84457eaea546541dc74c67cd1e4 [qa] Add a test for merkle proof malleation (Suhas Daftuar)
ed82f1700006830b6fe34572b66245c1487ccd29 have verifytxoutproof check the number of txns in proof structure (Gregory Sanders)

Pull request description:

  Recent publication of a weakness in Bitcoin's merkle tree construction demonstrates many SPV applications vulnerable to an expensive to pull off yet still plausible attack: https://bitslog.wordpress.com/2018/06/09/leaf-node-weakness-in-bitcoin-merkle-tree-design/

  This change would at least allow `verifytxoutproof` to properly validate that the proof matches a known block, with known number of transactions any time after the full block is processed. This should neuter the attack entirely.

  The negative is that a header-only processed block/future syncing mode would cause this to fail until the node has imported the data required.

  related: #13451

  `importprunedfunds` needs this check as well. Can expand it to cover this if people like the idea.

Tree-SHA512: 0682ec2b622a38b29f3f635323e0a8b6fc071e8a6fd134c954579926ee7b516e642966bafa667016744ce49c16e19b24dbc8801f982a36ad0a6a4aff6d93f82b

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