Make `commit_reversed_operations` more capable.

Right now, this method (known as `commit_undo_ops` before #372) just checks that the latest set of un-synchronized operations in the replica match the operations to undo. If that's the case, it removes them from the list of un-synchronized operations, reverses their effect on the tasks, and declares success. Otherwise, it fails.

This could be more robust -- if an operation does not precisely match the most recently recorded local operation, there's enough data to create a _new_, reversed operation (switch Create <--> Delete, swap `value` and `old_value` in Update) and commit that. Adding such support would be a big step on the way to allowing "undo" _after_ a sync has occurred.

I think we should fail the `commit_reversed_operations` if the reversed operation doesn't apply cleanly. So:
- A reversed Create is a Delete, but if the task doesn't exist, that should fail.
- A reversed Delete is a Create, but if the task still exists, that should fail.
- A reversed Update specifies the `old_value` it expects to find, but if the value is different, it should fail.