Add caching of local addresses

Adds a 5 second cache of local addresses.

There is room for refactoring the NetworkUtils class further
to avoid the functional overlap with the existing static
enumeration of interfaces.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7600/