Add fast path to OpSet.getMissingChanges
Add fast path to OpSet.getMissingChanges

Improves performance in the common case where user wants to just get the
most recent couple of changes, e.g. in the context of a frequent sync.
In this case, the cost becomes linear in the number of changes returned,
whereas previously it was linear in the size of the entire change
history.

In [this benchmark](https://gist.github.com/pvh/73473686865b9e421b620bd308fc9c58),
the case of 1,000 changes and syncing after each change speeds up by
roughly 10x after applying this patch.

automerge/automerge,automerge/automerge