allow multiple approvers to avoid bottlenecks

[ralfharing]

Right now, one approver can effectively lock down a changeset by assigning himself to it. If a user makes changes based upon the approver's comments, only that approver can sign off on them even though many others might be working on the queue and could easily verify that the changes were made and sign off on them themselves. This is particularly problematic if the approver happens not to be around for a while. Then there is a very poor user experience as the changes languish when they don't need to.

After sending a changeset back, the approver should be decoupled from the changeset. Then when the user submits it can be treated in a first-in-first-out manner along with all the other changes. Whoever sees it first can approve it. (They'll obviously see the previous comments and can make their own judgement about whether they want to let it wait. Some changes will surely be best handled that way. But right now we needlessly handicap the normal workflow.)