Data server_tokens.map_idx_to_media.tokens_image.batch_f32 is read exactly once,
by mtmd_encode, however it was retained as long as the input image was present
in the sequence. Add a manual free function to clear out this data after encoding.
Solves:
* Memory wasted in struct server_tokens
* The same wasted memory in the ram cache
* Long copy durations cloning this data to/from ram cache
* Accounting failures in ram cache (`batch_f32` can be larger than a sequence's entire KV)
* The above accounting failures leading to terminal memory leaks in pathological cases
* Remove JSON serialization for `batch_32` which was unused, and had no foreseeable usecase
d3e86a5431