Fix python map.items() racing with bpf delete

Fix python map.items() racing with bpf delete

The default python implementation of itervalues(), iteritems(), items(), and
values() do a sequence of iter().next followed by self[key], which may
produce a KeyError since a bpf program can delete them quickly. It
should be safe to suppress such errors, hiding them from the user.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>