Proxy: remove dependency on Lwt_unix.sleep

Context

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

Short term, we just make the function take an extra arg.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR