server: improve speed of speculative decoding

[ngxson]

Fix #12968

I'm testing with:

• draft model: https://huggingface.co/unsloth/Qwen3-0.6B-GGUF (using Q8_0)
• main model: https://huggingface.co/unsloth/Qwen3-8B-GGUF (using Q4_K_M)

So far the results are coherent.

How it works:

[ngxson]

server tests passed locally, this should be ready for review @ggerganov