sql_browser: set a server timing header to publish query time

https://w3c.github.io/server-timing/#the-server-timing-header-field

bt5/erp5_sql_browser/SkinTemplateItem/portal_skins/erp5_sql_browser/ZMySQLDAConnection_getQueryResultAsJSON.py

 from decimal import Decimal
 import datetime
+import time
 import json
 from DateTime import DateTime
 
 response = container.REQUEST.RESPONSE
-
+start = time.time()
 try:
   results = context.manage_test(query)
   data = [ results.names() ]
@@ -31,5 +32,6 @@ for line in data[1:]:
     new_line.append(v)
   new_data.append(new_line)
 
+response.setHeader("Server-Timing", "db=%s;" % (time.time() - start))
 response.setHeader('Content-Type', 'application/json')
 return json.dumps(new_data, indent=2)