libostree: Check out directories depth-first in serial, switch to sync API

The way we recurse into subdirectories in parallel makes it far too
easy to hit up against the arbitrary Linux fd limit of 1024.

Since the fix here is about dropping parallelism, let's just go all
the way for now and make a plain old synchronous API =(

This does simplify both internal callers which wanted a sync API
anyways.

https://bugzilla.gnome.org/show_bug.cgi?id=706380
doc: Add repo docs