BUG/MINOR: deviceatlas: consume a scheduled update before loading it

BUG/MINOR: deviceatlas: consume a scheduled update before loading it

The update flag in the shared map was only cleared after a successful
reload. When the published data file could not be opened, compiled or
loaded, every following request retried it: an unreadable file produced
one alert per request, flooding the logs and lowering the throughput,
and a file failing to compile was compiled again on each request. This
is the case for example since the automatic chroot became the default,
as the data file path is no longer reachable at run time.

The flag is now cleared as soon as the path has been read, so a failing
file is reported once and retried only when the scheduler publishes a
new one. This also avoids losing an update published while the previous
one was being compiled.

This should be backported to lower branches.