Replace std::string PaymentServer::mapNetworkIdToName and CChainParams::...

...RPCisTestNet with CChainParams::NetworkIDString

related to #4326 and #3824
Please don't change the `getinfo` call, too many people rely on its current behavior. As it's a random grabbag of information from different parts, it's going to be deprecated at some point in favor of the more specific get*info calls.