bug fix for issues #293 evdns_base_load_hosts doesn't remove outdated addresses 

As mentioned at https://sourceforge.net/p/levent/bugs/293/
created a small function "evdns_base_discard_previous_requests" which discards 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.