server : context checkpointing for hybrid and recurrent models (#16382)

* initial commit for branch 3

* generalize `swa_checkpoint` to `ctx_checkpoint`

this extends `llama-server`'s SWA checkpointing logic to include
hybrid/recurrent models such as Jamba, Granite

* oops

* disable debug prints

* keep backwards compat with `--swa-checkpoints`

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

* update prompt re-processing message

* fix off-by-one error per GG

* keep `seq_rm` log per GG

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

* server : fix checkpoint logic to support recurrent caches

* server : cleanup and fixes

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