core: fix potential crash on service name collision

core: fix potential crash on service name collision

If there is a service name collision and the entry group callback calls
avahi_s_entry_group_reset or avahi_s_entry_group free on the group in
question, the entries were released. This could cause a crash in
withdraw_rrset as it is walking a list of entries at this time.

The fix for this issue is to schedule a cleanup event to clean up
entries after a a short timeout (currently one second). If a cleanup
occurs for any other reason the event is cancelled.

http://avahi.org/ticket/302