RPC: FIX Cohttp closing client connection

What

Cohttp allows to terminate the relay connection created on the client side.

Why

Cohttp is not closing client-side connection on request on relay connections.

How

Cohttp now exposes close_fn provided by Cohttp_lwt.Client.call. This callback should be used when a newly created connection should be closed.

Manually testing the MR

In Tezos, described in https://docs.google.com/document/d/1dqOec9Fm8D9AuZOg6-7UO4-7P26vpHxXiFFUJsoOEDg/edit#heading=h.rkbp5frwoc0y

Requires adaptation in Resto and RPC middleware.

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