v17: Add support for blockchain methods

[tcharding]

Add support to the v17 client and integration tests for blockchain section methods.

//! **== Blockchain ==**
//! - [x] `getbestblockhash`
//! - [x] `getblock "blockhash" ( verbosity ) `
//! - [x] `getblockchaininfo`
//! - [x] `getblockcount`
//! - [x] `getblockhash height`
//! - [x] `getblockheader "hash" ( verbose )`
//! - [x] `getblockstats hash_or_height ( stats )`
//! - [x] `getchaintips`
//! - [x] `getchaintxstats ( nblocks blockhash )`
//! - [x] `getdifficulty`
//! - [x] `getmempoolancestors txid (verbose)`
//! - [x] `getmempooldescendants txid (verbose)`
//! - [x] `getmempoolentry txid`
//! - [x] `getmempoolinfo`
//! - [x] `getrawmempool ( verbose )`
//! - [x] `gettxout "txid" n ( include_mempool )`
//! - [x] `gettxoutproof ["txid",...] ( blockhash )`
//! - [x] `gettxoutsetinfo`
//! - [x] `preciousblock "blockhash"`
//! - [-] `pruneblockchain`
//! - [-] `savemempool`
//! - [-] `scantxoutset <action> ( <scanobjects> )`
//! - [x] `verifychain ( checklevel nblocks )`
//! - [-] `verifytxoutproof "proof"`
//!
//! Key:
//! - `[ ]` means not yet done.
//! - `[x]` marks means implemented _and_ tested.
//! - `[-]` means it was considered and intentionally not done.