Merge #10857: [RPC] Add a deprecation warning to getinfo's output

1c9b818 getinfo deprecation warning (Andrew Chow)

Pull request description:

  This is an alternative to #10841

  This PR implements @gmaxwell's suggestion of a `nag` field for getinfo which warns about the deprecation. Instead of calling it `nag`, I have named it `deprecation-warning`. The output of `getinfo` will look like this:
  ```
  {
    "version": 149900,
    "protocolversion": 70015,
    "walletversion": 139900,
    "balance": 0.00000000,
    "blocks": 476281,
    "timeoffset": 0,
    "connections": 2,
    "proxy": "",
    "difficulty": 804525194568.1318,
    "testnet": false,
    "keypoololdest": 1496858803,
    "keypoolsize": 197,
    "unlocked_until": 0,
    "paytxfee": 0.00000000,
    "relayfee": 0.00001000,
    "errors": "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications",
    "deprecation-warning": "WARNING: getinfo is deprecated and will be fully removed in 0.16. Projects should transition to using getblockchaininfo, getnetworkinfo, and getwalletinfo before upgrading to 0.16"
  }
  ```

  I think this should be tagged for 0.15

Tree-SHA512: ea1bac96a67f797519e8748ddd661cf0a1127cbc38f145b98f10cf9b54dcf0519b353062ce9888e1f51875497299c75ff5147566944451bc3fc117620e773489

src/rpc/misc.cpp

@@ -51,6 +51,7 @@ UniValue getinfo(const JSONRPCRequest& request)
             "\nDEPRECATED. Returns an object containing various state info.\n"
             "\nResult:\n"
             "{\n"
+            "  \"deprecation-warning\": \"...\" (string) warning that the getinfo command is deprecated and will be removed in 0.16\n"
             "  \"version\": xxxxx,           (numeric) the server version\n"
             "  \"protocolversion\": xxxxx,   (numeric) the protocol version\n"
             "  \"walletversion\": xxxxx,     (numeric) the wallet version\n"
@@ -58,7 +59,7 @@ UniValue getinfo(const JSONRPCRequest& request)
             "  \"blocks\": xxxxxx,           (numeric) the current number of blocks processed in the server\n"
             "  \"timeoffset\": xxxxx,        (numeric) the time offset\n"
             "  \"connections\": xxxxx,       (numeric) the number of connections\n"
-            "  \"proxy\": \"host:port\",     (string, optional) the proxy used by the server\n"
+            "  \"proxy\": \"host:port\",       (string, optional) the proxy used by the server\n"
             "  \"difficulty\": xxxxxx,       (numeric) the current difficulty\n"
             "  \"testnet\": true|false,      (boolean) if the server is using testnet or not\n"
             "  \"keypoololdest\": xxxxxx,    (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
@@ -66,7 +67,7 @@ UniValue getinfo(const JSONRPCRequest& request)
             "  \"unlocked_until\": ttt,      (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"
             "  \"paytxfee\": x.xxxx,         (numeric) the transaction fee set in " + CURRENCY_UNIT + "/kB\n"
             "  \"relayfee\": x.xxxx,         (numeric) minimum relay fee for transactions in " + CURRENCY_UNIT + "/kB\n"
-            "  \"errors\": \"...\"           (string) any error messages\n"
+            "  \"errors\": \"...\"             (string) any error messages\n"
             "}\n"
             "\nExamples:\n"
             + HelpExampleCli("getinfo", "")
@@ -85,6 +86,8 @@ UniValue getinfo(const JSONRPCRequest& request)
     GetProxy(NET_IPV4, proxy);
 
     UniValue obj(UniValue::VOBJ);
+    obj.push_back(Pair("deprecation-warning", "WARNING: getinfo is deprecated and will be fully removed in 0.16."
+        " Projects should transition to using getblockchaininfo, getnetworkinfo, and getwalletinfo before upgrading to 0.16"));
     obj.push_back(Pair("version", CLIENT_VERSION));
     obj.push_back(Pair("protocolversion", PROTOCOL_VERSION));
 #ifdef ENABLE_WALLET