server : simplify state machine for slot (#9283)

* server : simplify state machine for slot

* add SLOT_STATE_DONE_PROMPT

* pop_deferred_task

* add missing notify_one

* fix passkey test

* metrics : add n_busy_slots_per_decode

* fix test step

* add test

* maybe fix AddressSanitizer?

* fix deque ?

* missing lock

* pop_deferred_task: also notify

* Update examples/server/server.cpp

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

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