rpc: add `getblockchaininfo` and `getnetworkinfo` #4127

Adds two new info query commands that take over information from hodge-podge getinfo.

Also some useful new information is added:

• getblockchaininfo
  • chain: (string) current chain (main, testnet3, regtest)
  • verificationprogress: (numeric) estimated verification progress
  • bestblockhash: (string) the hash of the currently best block
  • chainwork
• getnetworkinfo
  • numblocksofpeers: (numeric) number of blocks of peers, as estimated from version messages; same as shown in the GUI
  • localaddresses: (array) local addresses, from mapLocalHost (fixes #1734)

Example getnetworkinfo:

{
    "version" : 99900,
    "protocolversion" : 70002,
    "timeoffset" : 0,
    "connections" : 8,
    "proxy" : "",
    "relayfee" : 0.00001000,
    "localaddresses" : [
        {
            "address" : "89.98.179.240",
            "port" : 8333,
            "score" : 4
        }
    ]
}

Example getblockchaininfo:

{
    "chain" : "main",
    "blocks" : 299320,
    "bestblockhash" : "00000000000000000fe7330173c917964dff6c763478a8ad7ee18998069bcfff",
    "difficulty" : 8000872135.96816349,
    "verificationprogress" : 0.99996298,
    "chainwork" : "0000000000000000000000000000000000000000000055887ab265d0a5977101"
}

This is analogous to getwalletinfo that was introduced by pull #3717.

Untested ACK.

I'd vote for adding best-block-hash to getblockchaininfo.

Ok, added bestblockhash.

The only field from getinfo that I don't know where to place otherwise is errors, as it reports errors from different sources

• anything that sets strMiscWarning (blockchain code sets this, as well as P2P network interaction in main)
• various block chain warnings
• pre-release build warning
• alerts

Instead of returning only the message with the highest priority, maybe it makes sense to add a getwarnings call that returns all active runtime warnings along with their priority?

ACK

Maybe adding nChainWork would be useful to expose here as well. (optional, not needed in this PR).

• Removed numblocksofpeers after discussion on IRC yesterday. Would be better to get rid of it in the GUI as well (and instead show the list of connected nodes with their raw values in the debug window?).

<gmaxwell> wumpus: I'm unsure about that numblocksofpeers... there really isn't a good way to indicate in the output "This figure is not authenticated in any way and you shouldn't depend on it for anything— it can display complete rubbish, and troublemakers have screwed with it in the past."
<wumpus> gmaxwell: right, I just want the same information available in the RPC interface in the GUI
<wumpus> gmaxwell: if we don't want it on the RPC interface, I'd want to remove it from the GUI too
<wumpus> gmaxwell: (which isn't too much of an issue as the progress display uses the progress estimate)
<gmaxwell> I don't think it's that terribly useful in either (now that we have the progress estimate). Getpeerinfo gives the per peer raw values, which are more useful.
<wumpus> ok, agreed, let's just remove it then
<sipa> gmaxwell, wumpus: ACK

• added chainwork

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