bug fix for issues #293 evdns_base_load_hosts doesn't remove outdated adresses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.
bug fix for issues #293 evdns_base_load_hosts doesn't remove outdated addresses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_flush_outdated_host_addresses" which removes all the previous host addresses, if user wants to clean up the list of hosts can call and use this function.
Defination of this function is part of another patch.
Merge remote-tracking branch 'origin/pr/86'
Rename flush_outdated_host_addresses to clear_host_addresses

"flush" can imply writing something out to a file or connection before
clearing it; "clear" always means "remove".  It's also potentially
misleading to say "outdated" here, since the function removes _all_
addresses regardless, not just certain outdated ones.

Also, don't free the lock in this function.  Also reindent the function.