Implement the getdifficulty RPC

[teor2345]

Motivation

This is one of the initial RPCs called by node-stratum-pool:
https://github.com/s-nomp/node-stratum-pool/blob/d86ae73f8ff968d9355bb61aac05e0ebef36ccb5/lib/pool.js#L387-L392

It is required to detect proof of work or proof of stake coins:
https://github.com/s-nomp/node-stratum-pool/blob/d86ae73f8ff968d9355bb61aac05e0ebef36ccb5/lib/pool.js#L424-L427

The pool won't run if this RPC returns an error:
https://github.com/s-nomp/node-stratum-pool/blob/d86ae73f8ff968d9355bb61aac05e0ebef36ccb5/lib/pool.js#L413-L416

Specifications

https://zcash.github.io/rpc/getdifficulty.html

Complex Code or Requirements

We have an existing state request for the difficulty, so we could just use that for now?
(That state request also gets the history tree, which we might want to change in a later PR, but only if it impacts performance.)

Testing

We can do a snapshot test and manually test with s-nomp or equihash-solomining.

Related Work

I'm pretty sure this blocks #5934 with s-nomp and equihash-solomining.