unbound-dhcp-leases-bridge: Replace leases file atomically

When there is a large number of leases, writing the file may
take a long time. When unbound is re-reading its configuration
in that time, the file might syntactically incorrect.

This change writes the file first and then moves it
to the right place in one transaction.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>