Fixed several bugs wth MESS sync

The MESS sync was not actually working effectively. This fix
fixes the following:

 - ajax-sync-member-and-account-with-MESS.php now correctly updates
   member and account information in database
 - ajax-sync-member-and-account-with-MESS.php now times out after a
   few seconds, which prevents it from blocking other async ajax
   calls from accessing the $_SESSION variable. The result at the
   register was that poll scaling stopped workin when the internet
   went out
 - The javascript that called ajax-sync-member-and-account-with-MESS.php
   in pos2.php now handles an empty json array more elegantly