We move `.jj/repo/op_heads/*` into `.jj/repo/op_heads/heads/*` almost
a year ago, in commits 90a66ec and 37ba175. We said we would
drop support for it in 0.9+. I think we said that before we started
doing monthly releases, but I we're still past the goal of 6 months
(which is what I think we were aiming for).

Consider how one would implment the current `OpHeadsStore` interface
for a cloud-based backend. After `OpHeadsStore::add_op_head()` is
called, the set of op heads temporarily contains two heads (typically)
until `OpHeadsStore::remove_op_head()` is called. That's not invalid,
but it's annoying to have to deal with that state more than
necessary. Also, it's unnecessarily inefficient to send the addition
and removal of op heads as separate RPCs. This patch therefore adds a
`update_op_heads()` method that takes a list of old heads to remove
and a single new head to add. Coming patches will start migrating to
that method.

`OpHeadsStoreLock::promote_new_op()` doesn't add much over the new
`update_op_heads()`, so let's switch to the latter.

I think the idea behind `handle_ancestor_ops()` was to let our backend
at Google delegate the work to the server, which could then avoid
walking ancestors. However, we're now thinking that we're going to
make our server resolve divergent operations on its own instead, so
the client will never see more than one op head, unless it manually
creates the second op head itself (e.g. because the user ran two
concurrent commands). In those cases it should be fine to do the
walk. So let's simplify the trait by removing the function.

This gets us closer to being able to use the new `update_op_heads()`
function here (without calling it multiple times).