libevent seems to have invaded `libbitcoin_common` via `url.cpp`

`url.cpp` is used only from `test/fuzz/string.cpp` and `wallet/rpcwallet.cpp`

libevent should only be required for bitcoind, bitcoin-cli and (due only to the hack of passing wallet names) bitcoin-qt

const UniValue& result = find_value(reply, "result");
                const UniValue& error  = find_value(reply, "error");
                if (!error.isNull()) {
                    
                    int code = error["code"].get_int();
                    if (fWait && code == RPC_IN_WARMUP)
                        throw CConnectionFailed("server in warmup");
                    strPrint = "error: " + error.write();
                    nRet = abs(code);
                    if (error.isObject())
                    {
                        UniValue errCode = find_value(error, "code");
                        UniValue errMsg  = find_value(error, "message");
                        strPrint = errCode.isNull() ? "" : "error code: "+errCode.getValStr()+"\n";
                        if (errMsg.isStr())
                            strPrint += "error message:\n"+errMsg.get_str();
                        if (errCode.isNum() && errCode.get_int() == RPC_WALLET_NOT_SPECIFIED) {
                            strPrint += "\nTry adding \"-rpcwallet=<filename>\" option to bitcoin-cli command line.";
                        }
                    }
                } else {
                    
                    if (result.isNull())
                        strPrint = "";
                    else if (result.isStr())
                        strPrint = result.get_str();
                    else
                        strPrint = result.write(2);
                }