kv-cache : rework kv_cell (#13706)
* kv-cache : rework kv_cell
ggml-ci
* kv-cells : use "shift" instead of "delta" consistently
ggml-ci
* llama : add llama_max_parallel_sequences()
ggml-ci
* kv-cells : update comments [no ci]
* context : fail upon construction if sequences exceed max value
ggml-ci
* kv-cells : get_pos() -> pos_get() + comments
ggml-ci
* kv-cells : fix tracking of "used" cells
ggml-ci
