pull: Lower max concurrent HTTP requests to 2

I believe this may help re-enabling the libcurl low-speed limit
checks in the case where we have a full 8 HTTP requests outstanding
with the remote server, but it's actually throttling us.

In practice, we really don't need more than two.  Experimenting
with this using the builtin Go HTTP server, this actually seems
to increase performance; but I get pretty wild timing variations
overall.

src/libostree/ostree-repo-private.h

@@ -33,7 +33,7 @@ G_BEGIN_DECLS
 #define _OSTREE_SUMMARY_CACHE_DIR "summaries"
 #define _OSTREE_CACHE_DIR "cache"
 
-#define _OSTREE_MAX_OUTSTANDING_FETCHER_REQUESTS 8
+#define _OSTREE_MAX_OUTSTANDING_FETCHER_REQUESTS 2
 #define _OSTREE_MAX_OUTSTANDING_DELTAPART_REQUESTS 2
 
 /* We want some parallelism with disk writes, but we also