refactor: (server) move `server_context::update_chat_msg` to HTTP thread

[ngxson]

update_chat_msg() calls common_chat_msg_diff::compute_diffs which can negatively impact the performance for text generation

we should find a way to move it into HTTP thread.

not 100% sure how the implement will look like, but I'm creating this issue so that it's not forgotten.

if any maintainers / contributors willing to take this task, please briefly mention your approach below