Expose closefn function

[gabrielmoise17]

This work has been done together by the following people:
@savvadia
@vect0r-vicall
@picojulien
@johnyob
@raphael-proust

Motivation

cohttp is not closing client-side connections on request in relay connections, which could lead to resource leak.
Exposing the close_fn function is already present in the newer v6 version, so this addition we think is necessary as it allows finer resource management and is likely to be a backport of the feature from v6 to v5.

Solution

cohttp now exposes close_fn defined by Cohttp_lwt.Client.call. This callback should be used when a newly created connection should be closed, thanks to the new Cohttp_lwt.Client.call_with_closefn function.