feat(tvix/store/nar-info): Support async blob upload

Right now all blob uploads are performed synchronously, this means if a
NAR contains many small files, the import time is dominated by round
trip time to the blob service. For small files, we can buffer them in
memory and upload them asynchronously to the blob service. Before
returning we make sure to join all the uploads to make sure they
complete successfully before responding OK.

This reduces time to import a bash-interactive closure on my machine
from 1m19s to 7s.

Change-Id: Ica3695c159e6c8ad8769281ac20d037e3143e856
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10679
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: Connor Brewster <cbrewster@hey.com>

tvix/nar-bridge/pkg/importer/roundtrip_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"sync"
 	"testing"
 
 	castorev1pb "code.tvl.fyi/tvix/castore-go"
@@ -27,6 +28,7 @@ func TestRoundtrip(t *testing.T) {
 	narContents, err := io.ReadAll(f)
 	require.NoError(t, err)
 
+	var mu sync.Mutex
 	blobsMap := make(map[string][]byte, 0)
 	directoriesMap := make(map[string]*castorev1pb.Directory)
 
@@ -41,7 +43,9 @@ func TestRoundtrip(t *testing.T) {
 			dgst := mustBlobDigest(bytes.NewReader(contents))
 
 			// put it in filesMap
+			mu.Lock()
 			blobsMap[base64.StdEncoding.EncodeToString(dgst)] = contents
+			mu.Unlock()
 
 			return dgst, nil
 		},