server : use std::move whenever possible (#12936)

* server : use std::move whenever possible

* use r-value ref

* Apply suggestions from code review

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* make task creation scoped

* restore std::move

* fix task_id not set correctly

* apply changes from suggestion

Co-authored-by: ggerganov <ggerganov@gmail.com>

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

Author: ngxson