Perform crypto store operations directly after transaction

At least on Safari but perhaps other browsers as well, you must perform
IndexedDB operations in the same JS task as you start the transaction. As a
concrete example, you cannot open the transaction and await some promise before
actually using it.

This fixes the crypto store to meet this requirement.

Fixes element-hq/element-web#12207

src/client.js

@@ -685,6 +685,9 @@ MatrixClient.prototype.initCrypto = async function() {
         throw new Error(`Cannot enable encryption: no cryptoStore provided`);
     }
 
+    logger.log("Crypto: Starting up crypto store...");
+    await this._cryptoStore.startup();
+
     // initialise the list of encrypted rooms (whether or not crypto is enabled)
     logger.log("Crypto: initialising roomlist...");
     await this._roomList.init();