server : refactor multitask handling (#9274)

* server : remove multitask from server_task

* refactor completions handler

* fix embeddings

* use res_ok everywhere

* small change for handle_slots_action

* use unordered_set everywhere

* (try) fix test

* no more "mutable" lambda

* Apply suggestions from code review

Co-authored-by: Georgi Gerganov <[email protected]>

* use deque

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Co-authored-by: Georgi Gerganov <[email protected]>