bug fix for issues #293 evdns_base_load_hosts doesn't remove outdated adresses

As mentioned at https://sourceforge.net/p/levent/bugs/293/
 created a small function "evdns_base_flush_outdated_host_addresses" which removes all the previous requests of hosts , if user wants to clean up the list of hosts can call and use this function. 
Requires function declaration to be added in include/event2/dns.h
 Adding it in another patch for the same bug.