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.