[changelog] Centralize pubsubconsumer assignment to synchronized collection

[ZacAttack]

[changelog] Centralize pubsubconsumer assignment to synchronized collection

getAssignment returns a new HashSet, but it's backed by the raw collection, making it non threadsafe. Tweaks here to make sure we don't hit any weird behavior or concurrent modification exceptions by serializing on the assignment.

Resolves #XXX

How was this PR tested?

Does this PR introduce any user-facing changes?

• [ ] No. You can skip the rest of this section.
• [ ] Yes. Make sure to explain your proposed changes and call out the behavior change.