Prevent ProxyServer Durable Object eviction

cloudflare/workerd#1138 introduced Durable Object's eviction behaviour
to `workerd`. We really don't want the `ProxyServer`'s singleton
object to be evicted, as this would invalidate proxy stubs' heap
addresses. This change makes sure the `preventEviction` flag is set.

packages/miniflare/src/plugins/core/index.ts

@@ -551,6 +551,12 @@ export function getGlobalServices({
           {
             className: "ProxyServer",
             uniqueKey: `${SERVICE_ENTRY}-ProxyServer`,
+            // `ProxyServer` relies on a singleton object containing of "heap"
+            // mapping addresses to native references. If the singleton object
+            // were evicted, addresses would be invalidated. Therefore, we
+            // prevent eviction to ensure heap addresses stay valid for the
+            // lifetime of the `workerd` process
+            preventEviction: true,
           },
         ],
         // `ProxyServer` doesn't make use of Durable Object storage

packages/miniflare/src/workers/core/proxy.worker.ts

@@ -62,11 +62,6 @@ function getType(value: unknown) {
 // TODO(someday): extract `ProxyServer` into component that could be used by
 //  other (user) Durable Objects
 export class ProxyServer implements DurableObject {
-  // On the first `fetch()`, start a `setInterval()` to keep this Durable Object
-  // and its heap alive. This is required to ensure heap references stay valid
-  // for the lifetime of this `workerd` process (except it isn't since `workerd`
-  // doesn't evict Durable Objects yet :P, but it probably will soon).
-  anchorInterval?: number;
   nextHeapAddress = ProxyAddresses.USER_START;
   readonly heap = new Map<number, unknown>();
 
@@ -116,8 +111,6 @@ export class ProxyServer implements DurableObject {
   }
 
   async fetch(request: Request) {
-    // Make sure this instance is kept alive
-    this.anchorInterval ??= setInterval(() => {}, 10_000);
     try {
       return await this.#fetch(request);
     } catch (e) {