unbound-dhcp-leases-bridge: Read configuration only once

This saves some resources when we re-read the same configuration file
too often.

Suggested-by: Anthony Heading <ajrh@ajrh.net>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

config/unbound/unbound-dhcp-leases-bridge

@@ -22,6 +22,7 @@
 import argparse
 import datetime
 import daemon
+import functools
 import ipaddress
 import logging
 import logging.handlers
@@ -444,6 +445,7 @@ class Lease(object):
 		return "localdomain"
 
 	@staticmethod
+	@functools.cache
 	def read_settings(filename):
 		settings = {}