Rework handling of monitored files.

From now a hash is used to store which files should be monitored
and to store their current cursor position. The entire hash is shared
between the main process and the worker threads.

A benefit of this is, to keep the current cursor position of each monitored file
during thread restarts and the hash is also designed to be re-generated in case of
a service reload without loosing any relevant data.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>