Fix server pool timeout
Fix server pool timeout

The timeout will not be triggered and there will be forever
pending requests, if the LDAP servers can not be reached.

Fixes #549

privacyidea/privacyidea,jh23453/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,jh23453/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,wheldom01/privacyidea,jh23453/privacyidea,jh23453/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,jh23453/privacyidea,privacyidea/privacyidea