improve output, group file changes within one second

The InMemoryAuthority print now only log events when the data are changed and
not when data are not changed. The change case is less frequently and a higher
meaning.

Rewriting the status file thought OpenVPN fires inotify events multiple times
(within one second). After this changes openvpn2dns waits until the status
file is unchanged for one second. So openvpn2dns reload the file content only
once.
config: correct exception message