repo: Make repo/tmp expiry configurable via tmp-expiry-seconds

We were arbitrarily only deleting content after exactly one day.  Some
use cases may want something else; make it configurable.

Closes: #170
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -98,6 +98,7 @@ struct OstreeRepo {
   OstreeRepoMode mode;
   gboolean enable_uncompressed_cache;
   gboolean generate_sizes;
+  guint64 tmp_expiry_seconds;
 
   OstreeRepo *parent_repo;
 };