api/stream: Improve reliability of /setactive API

[victorges]

What does this pull request do? Explain your changes. (required)
This is to do a couple improvements in the /setactive API, specifically made for improving
reliability of it. This is especially important for our stream nuking APIs, since for those we need
a reliable source of "where is the stream?". In the worst case we can start fetching this info
from the Stream Health API, but I believe a very simple change to the /setactive API should
already make this a lot more reliable, especially against malicious actors like our soccer
streamers who have learned how to explore the bugs in that API.

Apart from that very simple change, inplemented here 8045d8d, also made a bunch of
improvements to the API performance, since I noticed from our metrics that it's already
running dangerously close to the mist-api-connector timeout a lot of the times: [grafana]

Also did a small fix in the /user/suspended API here, just to avoid re-emailing the same user
that has already been suspended.

Specific updates (required)

• Avoid responding 403 to a /setactive: false request for a suspended stream (only when active=true)
• Ignore /setactive calls from sessions that are not the last one
• Optimize API handler execution by moving Webhooks and aux DB updates to background
• Only send account suspended emails to not already suspended users

-

• How did you test each of these updates (required)
  yarn test
  also deployed to staging and checked things still work

Does this pull request close any open issues?
It is related to but does not close https://github.com/livepeer/livepeer-infra/issues/830

I'm hoping that this change will make a full backend calling the kube-api unnecessary.
Our stream termination logic from the API should work perfectly.

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

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[hjpotter92]

lgtm!