Convert secret storage to new account data API

This converts all secret storage to use a newer account data API which uses
cached data in stored when available, but also knows how to ask the homeserver
in case it's invoked during early client startup before the initial sync.

As a consequence, it means most secret storage APIs are now async.

Part of https://github.com/vector-im/riot-web/issues/11901

src/client.js

@@ -498,6 +498,18 @@ MatrixClient.prototype.getSyncStateData = function() {
     return this._syncApi.getSyncStateData();
 };
 
+/**
+ * Whether the initial sync has completed.
+ * @return {boolean} True if at least on sync has happened.
+ */
+MatrixClient.prototype.isInitialSyncComplete = function() {
+    const state = this.getSyncState();
+    if (!state) {
+        return false;
+    }
+    return state === "PREPAED" || state === "SYNCING";
+};
+
 /**
  * Return whether the client is configured for a guest account.
  * @return {boolean} True if this is a guest access_token (or no token is supplied).
@@ -1944,14 +1956,23 @@ MatrixClient.prototype.getAccountData = function(eventType) {
 
 /**
  * Get account data event of given type for the current user. This variant
- * bypasses the local store and gets account data directly from the homeserver,
- * which can be useful very early in startup before the initial sync.
+ * gets account data directly from the homeserver if the local store is not
+ * ready, which can be useful very early in startup before the initial sync.
  * @param {string} eventType The event type
  * @return {module:client.Promise} Resolves: The contents of the given account
  * data event.
  * @return {module:http-api.MatrixError} Rejects: with an error response.
  */
-MatrixClient.prototype.getAccountDataFromServer = function(eventType) {
+MatrixClient.prototype.getAccountDataFromServer = async function(eventType) {
+    if (this.isInitialSyncComplete()) {
+        const event = this.store.getAccountData(eventType);
+        if (!event) {
+            return null;
+        }
+        // The network version below returns just the content, so this branch
+        // does the same to match.
+        return event.getContent();
+    }
     const path = utils.encodeUri("/user/$userId/account_data/$type", {
         $userId: this.credentials.userId,
         $type: eventType,