start/close streams in parallel on new worker metadata

[m-trieu]

Previously, StreamingEngineClient would start and close streams after getting new worker metadata synchronously (close 1 stream at time, open 1 stream at a time).

Make this happen in parallel and run async only blocking when needed.

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[m-trieu]

R: @scwhittle

[github-actions[bot]]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[ahmedabu98]

Hey @m-trieu, can you resolve some of the merge conflicts then tag Sam again?