Control Panel fix for issue 4302 (Unexpected errors using single JNDI connection with SSL and multiple threads.)

This fix has two parts:

1. Modify the threading mechanism used to search the contents of an entry.  From now only one thread is created to do this task.  The result is that there is no need of interrupting threads and that the number of threads that are generated is limited.

2. When the server contents are read, check also if the user connection (the one used by the LDAP browser to read user data) works.  If it is not the case, a message informing the user to provide authentication is displayed.  This modification helps in the case we encounter other issues and does not force the user to reopen the control panel dialog (or to figure out that the workaround is to go to 'File->Server to Administer'.

This check is already done for the admin connection, so this is just a completion of that check.