add cache module to store shadow and status data

cache.js

+module.exports = { getStatus, getShadow, setStatus, setShadow, clear, clearAll }
+
+const LRU = require("lru-cache");
+let cache = new LRU({
+    max: 500
+});
+
+function getStatus(deviceid) {
+	return cache.get(`${deviceid}:status`);
+}
+
+function getShadow(deviceid) {
+	return cache.get(`${deviceid}:shadow`);
+}
+
+function setStatus(deviceid, data) {
+	return cache.set(`${deviceid}:status`, data);
+}
+
+function setShadow(deviceid, data) {
+	return cache.set(`${deviceid}:shadow`, data);
+}
+
+function clear(deviceid) {
+	cache.del(`${deviceid}:status`);
+	cache.del(`${deviceid}:shadow`);
+}
+
+function clearAll() {
+	cache.reset();
+}