fix(swingset): recreateDynamicVat() waits for vat creation

We have three functions in `loadVat.js` to create a vat.
`createVatDynamically()`is used to create a dynamic vat for the first time.
It does not wait for the vat to be created, but instead returns the
newly-allocated vatID synchronously, so the vatAdminVat which invoked it (via
a device) can learn the vatID to wait for.

recreateStaticVat() and recreateDynamicVat() are used during kernel reboot to
reload vats created by the earlier instance. recreateStaticVat() returns a
Promise that fires when the vat is fully loaded (which includes evaluating
the source bundle). recreateDynamicVat() was not returning a Promise, rather
it followed the pattern of `createVatDynamically()` and returned a
synchronous vatID. The code in `kernel.js` which called it was using `await`,
as if it returned a Promise.

As a result, rebooting a kernel with multiple dynamic vats would attempt to
reload those vats in parallel. These vat reloads currently each take
~20s (for a dynamic ZCF vat with metering enabled), and are CPU bound, and
the engine isn't multithreaded, so they do not benefit from parallelism (this
will change when we have each vat in a separate subprocess). Also, the
accidental parallelism interferes with attempts to measure the time each
reload takes (to guide optimization efforts).

This changes `recreateDynamicVat` to return the same sort of Promise that
`recreateStaticVat` does.

closes #2871

Author: warner

packages/SwingSet/src/kernel/loadVat.js

@@ -55,13 +55,11 @@ export function makeVatLoader(stuff) {
    * @param {*} source  The source object implementing the vat
    * @param {*} dynamicOptions  Options bag governing vat creation
    *
-   * @returns {string}  The vatID of the vat
+   * @returns {Promise<void>} fires when the vat is ready for messages
    */
   function recreateDynamicVat(vatID, source, dynamicOptions) {
     // eslint-disable-next-line no-use-before-define
-    create(vatID, source, dynamicOptions, false, true);
-    // again we ignore create()'s Promise
-    return vatID;
+    return create(vatID, source, dynamicOptions, false, true);
   }
 
   /**