Bounce updates for placement and scaling

[ssalinas]

A few goals/changes in this PR:

• At the end of an expiring scale, if a request has bounceAfterScale set to true, bounce the request
• Configurably allow bounces to reuse hosts already running the task (for requests with SEPARATE placement). This would allow a new task from the bounce to run on the same host as an old task, but not the same host as another new task from the bounce. Accomplished this by adding the actionId to the pending task so it is more readily available when deciding placement.
• Let the backend handle bounce after scale rather than the ui issuing two calls. The new bounce after scale on the backend will now not spin up extra instances. The request is saved, but instead of enqueuing a pending request with UPDATED, we add a BOUNCE one instead.

Needs a good bit of testing still, but should hopefully solve a number of the bounce-related woes we've seen