Add unlimitedStorage to Firefox manifest; add timeout to IndexedDB access

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/416

The Chromium version of uBO has declared `unlimitedStorage` since the
extension was first published in 2014. Declaring this permission in
Firefox brings uBO inline with the Chromium version. I suspect some
reported errors could be caused by IndexedDB eviction due to the lack
of `unlimitedStorage` permission.

Additionally, a timeout has been added when uBO tries to access its
indexedDB storage. It's unclear whether this will help with the
mentioned related issue though, the root cause is still to be
identified.

platform/firefox/manifest.json

@@ -74,6 +74,7 @@
     "privacy",
     "storage",
     "tabs",
+    "unlimitedStorage",
     "webNavigation",
     "webRequest",
     "webRequestBlocking",

src/js/cachestorage.js

@@ -226,19 +226,30 @@
                     table.createIndex('value', 'value', { unique: false });
                 };
                 req.onsuccess = function(ev) {
+                    if ( resolve === undefined ) { return; }
                     req = undefined;
                     db = ev.target.result;
                     db.onerror = db.onabort = genericErrorHandler;
                     dbPromise = undefined;
                     resolve(db);
+                    resolve = undefined;
                 };
                 req.onerror = req.onblocked = function() {
+                    if ( resolve === undefined ) { return; }
                     req = undefined;
                     console.log(this.error);
                     db = null;
                     dbPromise = undefined;
                     resolve(null);
+                    resolve = undefined;
                 };
+                setTimeout(( ) => {
+                    if ( resolve === undefined ) { return; }
+                    db = null;
+                    dbPromise = undefined;
+                    resolve(null);
+                    resolve = undefined;
+                }, 5000);
             });
             return dbPromise;
         };
@@ -246,8 +257,8 @@
         const getFromDb = function(keys, keyvalStore, callback) {
             if ( typeof callback !== 'function' ) { return; }
             if ( keys.length === 0 ) { return callback(keyvalStore); }
-            let promises = [];
-            let gotOne = function() {
+            const promises = [];
+            const gotOne = function() {
                 if ( typeof this.result !== 'object' ) { return; }
                 keyvalStore[this.result.key] = this.result.value;
                 if ( this.result.value instanceof Blob === false ) { return; }
@@ -262,7 +273,7 @@
             };
             getDb().then(db => {
                 if ( !db ) { return callback(); }
-                const transaction = db.transaction(STORAGE_NAME);
+                const transaction = db.transaction(STORAGE_NAME, 'readonly');
                 transaction.oncomplete =
                 transaction.onerror =
                 transaction.onabort = ( ) => {
@@ -272,11 +283,13 @@
                 };
                 const table = transaction.objectStore(STORAGE_NAME);
                 for ( const key of keys ) {
-                    let req = table.get(key);
+                    const req = table.get(key);
                     req.onsuccess = gotOne;
                     req.onerror = noopfn;
-                    req = undefined;
                 }
+            }).catch(reason => {
+                console.info(`cacheStorage.getFromDb() failed: ${reason}`);
+                callback();
             });
         };