refactor(tvix/store): Asyncify PathInfoService and DirectoryService

We've decided to asyncify all of the services to reduce some of the
pains going back and for between sync<->async. The end goal will be for
all the tvix-store internals to be async and then expose a sync
interface for things like tvix eval io.

Change-Id: I97c71f8db1d05a38bd8f625df5087d565705d52d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9369
Autosubmit: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/store/src/tests/import.rs

@@ -111,10 +111,12 @@ async fn complicated() {
     // ensure DIRECTORY_WITH_KEEP and DIRECTORY_COMPLICATED have been uploaded
     assert!(directory_service
         .get(&DIRECTORY_WITH_KEEP.digest())
+        .await
         .unwrap()
         .is_some());
     assert!(directory_service
         .get(&DIRECTORY_COMPLICATED.digest())
+        .await
         .unwrap()
         .is_some());