fix: load the tags manifest asynchronously (vercel#64563)

## What?
A small update to FileSystemCache to replace sync calls with async.

## Why?

`loadTagsManifest` may be called multiple times per request. Since
`loadTagsManifest` is synchronous it blocks the main thread whilst
reading from the file system which could impact server performance.
Replacing these sync calls with async has no impact for consumers of the
FileSystemCache.

Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next/src/server/lib/incremental-cache/file-system-cache.ts

@@ -94,13 +94,34 @@ export default class FileSystemCache implements CacheHandler {
         'fetch-cache',
         'tags-manifest.json'
       )
-      this.loadTagsManifest()
+
+      this.loadTagsManifestSync()
     }
   }
 
   public resetRequestCache(): void {}
 
-  private loadTagsManifest() {
+  /**
+   * Load the tags manifest from the file system
+   */
+  private async loadTagsManifest() {
+    if (!this.tagsManifestPath || !this.fs || tagsManifest) return
+    try {
+      tagsManifest = JSON.parse(
+        await this.fs.readFile(this.tagsManifestPath, 'utf8')
+      )
+    } catch (err: any) {
+      tagsManifest = { version: 1, items: {} }
+    }
+    if (this.debug) console.log('loadTagsManifest', tagsManifest)
+  }
+
+  /**
+   * As above, but synchronous for use in the constructor. This is to
+   * preserve the existing behaviour when instantiating the cache handler. Although it's
+   * not ideal to block the main thread it's only called once during startup.
+   */
+  private loadTagsManifestSync() {
     if (!this.tagsManifestPath || !this.fs || tagsManifest) return
     try {
       tagsManifest = JSON.parse(
@@ -129,7 +150,7 @@ export default class FileSystemCache implements CacheHandler {
     // we need to ensure the tagsManifest is refreshed
     // since separate workers can be updating it at the same
     // time and we can't flush out of sync data
-    this.loadTagsManifest()
+    await this.loadTagsManifest()
     if (!tagsManifest || !this.tagsManifestPath) {
       return
     }
@@ -289,7 +310,7 @@ export default class FileSystemCache implements CacheHandler {
       }
 
       if (cacheTags?.length) {
-        this.loadTagsManifest()
+        await this.loadTagsManifest()
 
         const isStale = cacheTags.some((tag) => {
           return (
@@ -309,7 +330,7 @@ export default class FileSystemCache implements CacheHandler {
     }
 
     if (data && data?.value?.kind === 'FETCH') {
-      this.loadTagsManifest()
+      await this.loadTagsManifest()
 
       const combinedTags = [...(tags || []), ...(softTags || [])]