introduce scs metadata importer

Summary:
Prefetching metadata for the entries in a tree when we fetch it saves us
an extra round trip to the server to fetch a blob when only the metadata
for that blob is fetched. (This can happen often while parsing targets in
builds)

This implements a custom metadata fetcher to fetch this data when we
fetch a tree from the server.

Reviewed By: chadaustin

Differential Revision: D22086639

fbshipit-source-id: 5fe31d375bf6f7376eb67496d553d6b4540fc0c9

eden/fs/config/EdenConfig.h

@@ -232,6 +232,15 @@ class EdenConfig : private ConfigSettingManager {
                                                     "http",
                                                     this};
 
+  /**
+   * Source Control Service (scs) tier
+   */
+  ConfigSetting<bool> useScs{"scs:use-mononoke-scs", false, this};
+
+  ConfigSetting<std::string> scsTierName{"scs:tier",
+                                         "mononoke-scs-server",
+                                         this};
+
   /**
    * How often the on-disk config information should be checked for changes.
    */