Formalize Workers into Trait

[insipx]

Device + Consent sync operate with LocalEvents, which are events global to a single client instance. Workers are spawned on client instantiation and do work for the life of the client. Right now we only have a single worker for sync, but its reasonable to foresee other possible workers in the future.

If this pattern becomes useful, it would be great to formalize workers into a trait, where workers can be dynamically registered to the client and spawned on instantiation together. This worker pattern so far is useful for separating concerns for tasks that don't have strict deadlines (like syncing)