ELECTRON-788: add clear cache functionality (#506)

* ELECTRON-788: clean cache on bad exits

A few customers have reported that the cache gets corrupted when the app is not exited cleanly. And, upon the next start, we get cache_read_failures.

This fix adds an extra check during startup and exit to check if the app was exited cleanly and deletes the corrupted cache folder accordingly during startup.

* ELECTRON-788: add menu item, use electron session cache clear logic

Adds a new menu item to clear cache and reload the app.
Also remove the old logic of deleting the cache folder manually and use electron session api to clear the cache.

* ELECTRON-788: refactor code

js/cacheHandler/index.js

@@ -0,0 +1,39 @@
+const fs = require('fs');
+const nodePath = require('path');
+const electron = require('electron');
+
+const log = require('../log.js');
+const logLevels = require('../enums/logLevels.js');
+
+const cacheCheckFilename = 'CacheCheck';
+const cacheCheckFilePath = nodePath.join(electron.app.getPath('userData'), cacheCheckFilename);
+
+function handleCacheFailureCheckOnStartup() {
+
+    return new Promise((resolve) => {
+
+        if (fs.existsSync(cacheCheckFilePath)) {
+            log.send(logLevels.INFO, `Cache check file exists, so not clearing cache!`);
+            fs.unlinkSync(cacheCheckFilePath);
+            resolve();
+        } else {
+            log.send(logLevels.INFO, `Cache check file does not exist, we are clearing the cache!`);
+            electron.session.defaultSession.clearCache(() => {
+                log.send(logLevels.INFO, `Cleared cache!`);
+                resolve();
+            });
+        }
+
+    });
+
+}
+
+function handleCacheFailureCheckOnExit() {
+    log.send(logLevels.INFO, `Clean exit! Creating cache check file!`);
+    fs.writeFileSync(cacheCheckFilePath, "");
+}
+
+module.exports = {
+    handleCacheFailureCheckOnStartup: handleCacheFailureCheckOnStartup,
+    handleCacheFailureCheckOnExit: handleCacheFailureCheckOnExit
+};