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

tests/unit/unit1305.c

@@ -128,6 +128,7 @@ UNITTEST_START
     abort_unless(rc == CURLE_OK, "data node creation failed");
     key_len = strlen(data_key);
 
+    data_node->inuse = 1; /* hash will hold the reference */
     nodep = Curl_hash_add(hp, data_key, key_len+1, data_node);
     abort_unless(nodep, "insertion into hash failed");
     /* Freeing will now be done by Curl_hash_destroy */