Make `commit_reversed_operations` more capable.

[djmitche]

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.