fix refreshing of obsolete dns cache entries

- cache entries must be also refreshed when they are in use
- have the cache count as inuse reference too, freeing timestamp == 0 special
  value
- use timestamp == 0 for CURLOPT_RESOLVE entries which don't get refreshed
- remove CURLOPT_RESOLVE special inuse reference (timestamp == 0 will prevent refresh)
- fix Curl_hostcache_clean - CURLOPT_RESOLVE entries don't have a special
  reference anymore, and it would also release non CURLOPT_RESOLVE references
- fix locking in Curl_hostcache_clean
- fix unit1305.c: hash now keeps a reference, need to set inuse = 1