Use connection cache instead of Net.ctx as Client.ctx

[madroach]

This is a proposal to make the semantics of connection cache and Net.ctx in the Cohttp_lwt.Client module clearer.
This change is essentially summarized in cohttp-lwt/src/client.mli:

 module Make (Connection : S.Connection) :
-  S.Client with type ctx = Connection.Net.ctx
+  S.Client with type ctx = S.call

This is just a proof-of-concept on how to fix #1074