lib_proxy: don't call Lwt_unix.sleep directly

Context

We want tezos-proxy to compile to JS so we need to remove the dependency on Lwt_unix.sleep. There are discussions to provide a backend agnostic sleep https://github.com/ocsigen/lwt/issues/924 but it's not clear how it will end.

This MR is an alternative to !4529 (closed). As the need for sleep is limited to the proxy-server case, this MR makes the Proxy_server value of the mode type bring the function, instead of asking all callers to provide it.

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