info: cached info message

[VJHack]

Instead of displaying the original response metrics when the completion was pulled from cache, we want to display metrics related to the cache. The completion info is shown in the following format:

C: number of elements currently in the cache / cache size
t: total FIM time

 ... world\n");     | C: 3 / 250 | t: 0.66 ms

This is what the info message looks like when there is a cache hit.

Changes in this PR:

• Changed info message for when there is a cache hit
• Optimized cache usage by only storing the content and not the other metrics. This helps reduce the size of the cache.

This suggestion was mentioned here: #18 (review)

[ggerganov]

The llama-server now has the option to filter response fields, so another idea for improvement it to use this feature to minimize the amount of information that is transported from the server to the client, by filtering just the fields that are needed by llama.vim:

ggml-org/llama.cpp#10819