Make NetworkLocationCache load asynchronously

NetworkLocationCache loads synchronously because refresh() calls load()
directly, if the value is missing from the cache. This change sets the
cache entry to the "root" location, and then refreshes it asynchronously
to retrieve the correct value

presto-main/src/main/java/com/facebook/presto/execution/scheduler/NetworkLocationCache.java

@@ -75,6 +75,8 @@ public Optional<NetworkLocation> get(HostAddress host)
     {
         NetworkLocation location = cache.getIfPresent(host);
         if ((location == null) && (negativeCache.getIfPresent(host) == null)) {
+            // Store a value in the cache, so that refresh() is done asynchronously
+            cache.put(host, new NetworkLocation());
             cache.refresh(host);
         }
         return Optional.ofNullable(location);