fix(swingset): use better async style, improve comment

The use of `await` inside a try/finally block would expose a subtle timing
issue (if `thunk()` throws synchronously, the finalizer would run in the same
turn as the thunk, rather than in a subsequent turn). Our recommended style
avoids conditional `await` for just that reason, so this updates
`runWithoutMeteringAsync` to use a better approach.

packages/SwingSet/src/kernel/dummyMeterControl.js

@@ -27,11 +27,11 @@ export function makeDummyMeterControl() {
 
   async function runWithoutMeteringAsync(thunk) {
     meteringDisabled += 1;
-    try {
-      return await thunk();
-    } finally {
-      meteringDisabled -= 1;
-    }
+    return Promise.resolve()
+      .then(() => thunk())
+      .finally(() => {
+        meteringDisabled -= 1;
+      });
   }
 
   // return a version of f that runs outside metering

packages/SwingSet/src/kernel/liveSlots.js

@@ -465,7 +465,8 @@ function build(
       if (type === 'object') {
         // Set.delete() metering seems unaffected by presence/absence, but it
         // doesn't matter anyway because deadSet.add only happens when
-        // finializers run, which happens deterministically
+        // finializers run, and we wrote xsnap.c to ensure they only run
+        // deterministically (during gcAndFinalize)
         deadSet.delete(slot);
         droppedRegistry.register(val, slot, val);
       }

packages/SwingSet/src/kernel/vatManager/supervisor-subprocess-xsnap.js

@@ -60,12 +60,12 @@ function makeMeterControl() {
     const limit = globalThis.currentMeterLimit();
     const before = globalThis.resetMeter(0, 0);
     meteringDisabled += 1;
-    try {
-      return await thunk();
-    } finally {
-      globalThis.resetMeter(limit, before);
-      meteringDisabled -= 1;
-    }
+    return Promise.resolve()
+      .then(() => thunk())
+      .finally(() => {
+        globalThis.resetMeter(limit, before);
+        meteringDisabled -= 1;
+      });
   }
 
   // return a version of f that runs outside metering