Add repository "summary" file and metalink support

For Fedora and potentially other distributions which use globally
distributed mirrors, metalink is a popular solution to redirect
clients to a dynamic set of mirrors.

In order to make metalink work though, it needs *one* file which can
be checksummed.  (Well, potentially we could explode all refs into the
metalink.xml, but that would be a lot more invasive, and a bit weird
as we'd end up checksumming the checksum file).

This commit adds a new command:

$ ostree summary -u

To regenerate the summary file.  Can only be run by one process at a
time.

After that's done, the metalink can be generated based on it, and the
client fetch code will parse and load it.

https://bugzilla.gnome.org/show_bug.cgi?id=729585

src/libostree/ostree-core.h

@@ -117,6 +117,15 @@ typedef enum {
 #define OSTREE_COMMIT_GVARIANT_STRING "(a{sv}aya(say)sstayay)"
 #define OSTREE_COMMIT_GVARIANT_FORMAT G_VARIANT_TYPE (OSTREE_COMMIT_GVARIANT_STRING)
 
+/**
+ * OSTREE_SUMMARY_GVARIANT_FORMAT:
+ *
+ * refs: a(s(taya{sv})) - Map of ref name -> (latest commit size, latest commit checksum, additional metadata), sorted by ref name
+ * extensions: a{sv} - Additional metadata, none defined at the current time
+ */
+#define OSTREE_SUMMARY_GVARIANT_STRING "(a(s(taya{sv}))a{sv})"
+#define OSTREE_SUMMARY_GVARIANT_FORMAT G_VARIANT_TYPE (OSTREE_SUMMARY_GVARIANT_STRING)
+
 /**
  * OstreeRepoMode:
  * @OSTREE_REPO_MODE_BARE: Files are stored as themselves; can only be written as root