refactor: update Onyx.remove and Onyx.clear to cache a null value

When we remove values from storage we cache that the value was removed from storage
The next time someone requests this value we can tell them right away it's null
and they don't need to wait to read this from storage

lib/Onyx.js

@@ -471,8 +471,8 @@ function disconnect(connectionID, keyToRemoveFromEvictionBlocklist) {
  * @return {Promise}
  */
 function remove(key) {
-    // Remove from cache
-    cache.drop(key);
+    // Cache the fact that the value was removed
+    cache.set(key, null);
 
     // Optimistically inform subscribers
     keyChanged(key, null);
@@ -639,7 +639,7 @@ function clear() {
         .then((keys) => {
             _.each(keys, (key) => {
                 keyChanged(key, null);
-                cache.drop(key);
+                cache.set(key, null);
             });
         })
         .then(AsyncStorage.clear)