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/

Including the coinbase in the txoutproof seems the most effective fix, however results in a significant efficiency downgrade. Transactors will not even know a priori what the size of their proof will be within a couple orders of magnitude, unless they use the mid-state of SHA2 as detailed in the blog post.

Some applications, like Elements blockchain platform that take SPV-style proofs have optional access to a bitcoind to verify these proofs of inclusion and check depth in the chain. Returning `CBlockIndex::nTx` would allow an extremely easy and compact way of checking the depth of the tree, with no additional overhead to the codebase, and works with pruned nodes.

`getblockheader` is arguably not the place for it, but as mentioned before, is a natural workflow for us checking depth of a block in a possibly pruned node.

We should also ensure that `verifytxoutproof` ends up validating this depth fact as well, but left this for another PR.

make check
test_runner.py
test_runner.py feature_pruning

./bitcoind
./bitcoin-cli help getblock
./bitcoin-cli help getblockheader