feat: cache Forex responses

lib/forex.js

@@ -1,19 +1,29 @@
 const _ = require('lodash/fp')
 const axios = require('axios')
+const mem = require('mem')
+
 const BN = require('./bn')
+const T = require('./time')
 
 const MAX_ROTATIONS = 5
 
-const getFiatRates = () => axios.get('https://bitpay.com/api/rates').then(response => response.data)
+const _getFiatRates = () => (
+  axios.get('https://bitpay.com/api/rates')
+    .then(response => response.data)
+)
+
+const getFiatRates = mem(_getFiatRates, {
+  maxAge: 6 * T.hours,
+  cacheKey: () => ''
+})
 
 const API_QUEUE = [
   { api: getBitPayFxRate, name: 'bitpay', fiatCodeProperty: 'code', rateProperty: 'rate' }
 ]
 
 function getBitPayFxRate (fiatCode, defaultFiatMarket, fiatCodeProperty, rateProperty) {
-  return axios.get('https://bitpay.com/rates')
-    .then(response => {
-      const fxRates = response.data.data
+  return getFiatRates()
+    .then(({ data: fxRates }) => {
       const defaultFiatRate = findCurrencyRates(fxRates, defaultFiatMarket, fiatCodeProperty, rateProperty)
       const fxRate = findCurrencyRates(fxRates, fiatCode, fiatCodeProperty, rateProperty).div(defaultFiatRate)
       return {