Change initial key backup to background

Alters the APIs used for initial key backup so that the actual upload happens in
the background after all session are marked for backup.

src/client.js

@@ -997,12 +997,16 @@ MatrixClient.prototype.sendKeyBackup = function(roomId, sessionId, version, data
     );
 };
 
-MatrixClient.prototype.backupAllGroupSessions = function(version) {
+/**
+ * Marks all group sessions as needing to be backed up and schedules them to
+ * upload in the background as soon as possible.
+ */
+MatrixClient.prototype.scheduleAllGroupSessionsForBackup = async function() {
     if (this._crypto === null) {
         throw new Error("End-to-end encryption disabled");
     }
 
-    return this._crypto.backupAllGroupSessions(version);
+    await this._crypto.scheduleAllGroupSessionsForBackup();
 };
 
 MatrixClient.prototype.isValidRecoveryKey = function(recoveryKey) {

src/crypto/index.js

@@ -988,15 +988,17 @@ Crypto.prototype.importRoomKeys = function(keys) {
 /**
  * Schedules sending all keys waiting to be sent to the backup, if not already
  * scheduled. Retries if necessary.
+ *
+ * @param {number} maxDelay Maximum delay to wait in ms. 0 means no delay.
  */
-Crypto.prototype.scheduleKeyBackupSend = async function() {
+Crypto.prototype.scheduleKeyBackupSend = async function(maxDelay = 10000) {
     if (this._sendingBackups) return;
 
     try {
-        // wait between 0 and 10 seconds, to avoid backup
+        // wait between 0 and `maxDelay` seconds, to avoid backup
         // requests from different clients hitting the server all at
         // the same time when a new key is sent
-        const delay = Math.random() * 10000;
+        const delay = Math.random() * maxDelay;
         await Promise.delay(delay);
         let numFailures = 0; // number of consecutive failures
         while (1) {
@@ -1124,7 +1126,11 @@ Crypto.prototype.backupGroupSession = async function(
     this.scheduleKeyBackupSend();
 };
 
-Crypto.prototype.backupAllGroupSessions = async function(version) {
+/**
+ * Marks all group sessions as needing to be backed up and schedules them to
+ * upload in the background as soon as possible.
+ */
+Crypto.prototype.scheduleAllGroupSessionsForBackup = async function() {
     const remaining = await this._cryptoStore.doTxn(
         'readwrite',
         [
@@ -1142,10 +1148,8 @@ Crypto.prototype.backupAllGroupSessions = async function(version) {
     );
     this.emit("crypto.keyBackupSessionsRemaining", remaining);
 
-    let numKeysBackedUp;
-    do {
-        numKeysBackedUp = await this._backupPendingKeys(KEY_BACKUP_KEYS_PER_REQUEST);
-    } while (numKeysBackedUp > 0);
+    // Schedule keys to upload in the background as soon as possible.
+    this.scheduleKeyBackupSend(0 /* maxDelay */);
 };
 
 /* eslint-disable valid-jsdoc */    //https://github.com/eslint/eslint/issues/7307