Drop the shared manifest lock in the FS backend.

On Linux and macOS, two file descriptors opened by the same process are
treated as if they were different processes for the purpose of locking.

src/backend_fs.go

@@ -11,7 +11,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"sync"
 	"time"
 )
 
@@ -263,10 +262,6 @@ func (fs *FSBackend) HasAtomicCAS(ctx context.Context) bool {
 	return fs.hasAtomicCAS
 }
 
-// Right now updates aren't very common, so this lock is essentially entirely uncontended.
-// If it ever becomes a bottleneck it should be replaced with a per-manifest lock.
-var sharedManifestLock = sync.Mutex{}
-
 type manifestLockGuard struct {
 	file *os.File
 }
@@ -282,7 +277,6 @@ func lockManifest(fs *os.Root, name string) (*manifestLockGuard, error) {
 		file.Close()
 		return nil, fmt.Errorf("flock(LOCK_EX): %w", err)
 	}
-	sharedManifestLock.Lock()
 	return &manifestLockGuard{file}, nil
 }
 
@@ -290,7 +284,6 @@ func (guard *manifestLockGuard) Unlock() {
 	if guard.file != nil {
 		FileUnlock(guard.file)
 		guard.file.Close()
-		sharedManifestLock.Unlock()
 	}
 }