We need the report in print_document and print options (needed in
printer_tray)
Move the 'skip_update' right in the browse, it prevents a loop

See https://github.com/odoo/odoo/issues/3644

Also, it helps to have the value set/read in context close to each
other.
Avoid to hits the database too many times to check if the list of
printers needs to be refreshed.

Keep the last update datetime in cache and invalidate this datetime if is
is older than POLL_INTERVAL.  Thus, one process won't hit the DB more
than 1 time every POLL_INTERVAL (10 seconds currently) to check if it
needs to update the list.
Refresh the list of printers every 15 seconds instead of 10