Allow OrderedList to be initialized with an existing NodeCache

[aploshay]

The current code seems a bit odd in that:

• node_cache only has accessor (read-only) ability, and only privately
• it's set with a nil guard in the initializer, but with no way to provide a value

So it seems like there was meant to be some way to pass in an initial value, but this wasn't fully implemented? In any case, we need that ability in pumpkin, and this provides it.

This change has already been pushed to the v11.0.0.rc8 branch of our fork, so that pumpkin pull request pumpkin pull request #186 can make use of it. This PR exists to facilitate review and discussion.

Note that the code for current active-fedora still contains the same initialization pattern (nil guard, with no way to pass a value) for @node_cache. Even if we're violating intended use of OrderedList by using it in a nested fashion, I'm still scratching my head over this setup.