rpc: Add PoWHash to getblockverbose result.

[chappjc]

This adds a PoWHash field to the RPC server's getblock (verbose) and getblockheader (verbose) responses.

pre-DCP0011

$ ./dcrctl --testnet getblock 000000b396bfeaa6ae6fa9e3cee441d7215191630bdaa9b979a872985caed727
{
  "hash": "000000b396bfeaa6ae6fa9e3cee441d7215191630bdaa9b979a872985caed727",
  "powhash": "000000b396bfeaa6ae6fa9e3cee441d7215191630bdaa9b979a872985caed727",
  "confirmations": 1408,
  "size": 4491,
  "height": 1170047,
  "version": 11,
...

post-DCP0011

$ ./dcrctl --testnet getblock c7da7b548a2a9463dc97adb48433c4ffff18c3873f7e2ae99338a990dae039f0
{
  "hash": "c7da7b548a2a9463dc97adb48433c4ffff18c3873f7e2ae99338a990dae039f0",
  "powhash": "000000ce3005dbe95398f16f682438e298ff6301cc691e5df58839bd90351e33",
  "confirmations": 1407,
  "size": 4327,
  "height": 1170048,
  "version": 11,

[chappjc]

Oh, the help. Will update. But concept ACK or no?

[jrick]

looks reasonable to me

[chappjc]

Also for getblockheader (verbose=true) to match.

[davecgh]

Thanks for the PR. It looks good overall. I identified a few nits inline.

Also, please add another commit after the main rpc: Add ... commit that updates the JSON-RPC API docs to match the new reality:

docs: Update JSON-RPC API for powhash.