rpc: Handle getinfo locally in bitcoin-cli w/ -getinfo

This adds the infrastructure `BaseRequestHandler` class that takes care
of converting bitcoin-cli arguments into a JSON-RPC request object, and
converting the reply into a JSON object that can be shown as result.

This is subsequently used to handle the `-getinfo` option, which sends
a JSON-RPC batch request to the RPC server with
`["getnetworkinfo", "getblockchaininfo", "getwalletinfo"]`,
and after reply combines the result into what looks like a `getinfo`
result.

There have been some requests for a client-side `getinfo` and this
is my PoC of how to do it. If this is considered a good idea
some of the logic could be moved up to rpcclient.cpp and
used in the GUI console as well.

Extra-Author: Andrew Chow <achow101@gmail.com>

src/rpc/protocol.h

@@ -97,5 +97,7 @@ bool GenerateAuthCookie(std::string *cookie_out);
 bool GetAuthCookie(std::string *cookie_out);
 /** Delete RPC authentication cookie from disk */
 void DeleteAuthCookie();
+/** Parse JSON-RPC batch reply into a vector */
+std::vector<UniValue> JSONRPCProcessBatchReply(const UniValue &in, size_t num);
 
 #endif // BITCOIN_RPCPROTOCOL_H