In jst_server, implement partial file transfers with response code 206

diff --git a/jst_server b/jst_server
index e3e0da2..57048ee 160000 (submodule)
--- a/jst_server
+++ b/jst_server
@@ -1 +1 @@
-Subproject commit e3e0da29a2bf97954d17595a8bec40401afd8cd6
+Subproject commit 57048ee3d0285de6059ab228bf80d73f3848fbb0