don't os.File.Sync() on mac CAS items

os.File.Sync() was inneffective on mac in go prior to 1.12. In 1.12 onwards
it is effective, but slow: golang/go#26650

This change disables Sync for CAS uploads on mac, because we can (potentially)
verify this later. Let's see if this helps performance on mac: #67 (while
being less risky than #68).

cache/disk/disk.go

@@ -10,13 +10,16 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"sync"
 
 	"github.com/buchgr/bazel-remote/cache"
 	"github.com/djherbis/atime"
 )
 
+const isMac = runtime.GOOS == "darwin"
+
 // lruItem is the type of the values stored in SizedLRU to keep track of items.
 // It implements the SizedItem interface.
 type lruItem struct {
@@ -236,8 +239,11 @@ func (c *diskCache) Put(kind cache.EntryKind, hash string, expectedSize int64, r
 		}
 	}
 
-	if err := f.Sync(); err != nil {
-		return err
+	shouldSync := !isMac || kind != cache.CAS
+	if shouldSync {
+		if err := f.Sync(); err != nil {
+			return err
+		}
 	}
 
 	if err := f.Close(); err != nil {