Ensure cache storage is selected before access

Possibly related issue:
https://github.com/uBlockOrigin/uBlock-issues/issues/2136

Also reported internally, steps to reproduce the issue fixed here:

- Open uBO's dashboard through 3-dot > Add-ons > uBO > Settings
- Bring forth "Filter lists" pane
  We want the tab to be already opened at next launch
- Quit Firefox for Android
- Launch Firefox for Android

Result:

Very long launch time, lists marked as out of date.

src/js/background.js

@@ -270,6 +270,10 @@ const µBlock = {  // jshint ignore:line
     uiAccentStylesheet: '',
 };
 
+µBlock.isReadyPromise = new Promise(resolve => {
+    µBlock.isReadyResolve = resolve;
+});
+
 µBlock.domainFromHostname = domainFromHostname;
 µBlock.hostnameFromURI = hostnameFromURI;
 

src/js/messaging.js

@@ -1565,7 +1565,9 @@ const onMessage = function(request, sender, callback) {
         });
 
     case 'getLists':
-        return getLists(callback);
+        return µb.isReadyPromise.then(( ) => {
+            getLists(callback);
+        });
 
     case 'getLocalData':
         return getLocalData().then(localData => {

src/js/start.js

@@ -63,13 +63,6 @@ import {
 
 /******************************************************************************/
 
-// Load all: executed once.
-
-(async ( ) => {
-// >>>>> start of private scope
-
-/******************************************************************************/
-
 vAPI.app.onShutdown = ( ) => {
     staticFilteringReverseLookup.shutdown();
     io.updateStop();
@@ -312,10 +305,10 @@ const onHiddenSettingsReady = async ( ) => {
     }
 
     // Maybe override default cache storage
-    const cacheBackend = await cacheStorage.select(
+    µb.supportStats.cacheBackend = await cacheStorage.select(
         µb.hiddenSettings.cacheStorageAPI
     );
-    ubolog(`Backend storage for cache will be ${cacheBackend}`);
+    ubolog(`Backend storage for cache will be ${µb.supportStats.cacheBackend}`);
 };
 
 /******************************************************************************/
@@ -379,6 +372,9 @@ const createDefaultProps = ( ) => {
 
 /******************************************************************************/
 
+(async ( ) => {
+// >>>>> start of async/await scope
+    
 try {
     ubolog(`Start sequence of loading storage-based data ${Date.now()-vAPI.T0} ms after launch`);
 
@@ -506,5 +502,7 @@ if ( selfieIsValid ) {
 }
 ubolog(`All ready ${µb.supportStats.allReadyAfter} after launch`);
 
-// <<<<< end of private scope
+µb.isReadyResolve();
+
+// <<<<< end of async/await scope
 })();