[#1992] fstree: Allow working in SYNC mode

Make O_SYNC the default and allow to opt-out explicitly.

Signed-off-by: Evgenii Stratonikov <evgeniy@morphbits.ru>

pkg/local_object_storage/blobstor/fstree/option.go

@@ -29,3 +29,9 @@ func WithPath(p string) Option {
 		f.RootPath = p
 	}
 }
+
+func WithNoSync(noSync bool) Option {
+	return func(f *FSTree) {
+		f.noSync = noSync
+	}
+}