Fix `Connection_cache.call` hanging on unreachable server * What The `Connection_cache.call` function could hang indefinitely when attempting to establish a connection to an unreachable server. This change resolves that issue by ensuring that the connection creation process is asynchronous and non-blocking. * Why Previously, the `Connection.create` function returns before the connection is established. When the returned connection is binded, something in the code of `get_connection` prevents the correct handling of the `ECONNREFUSED` Unix error. * How The `create` helper function now uses `Connection.connect` (which returns an Lwt promise) and binds its result using `>>=`. This transforms the `create` function itself into an Lwt promise. Consequently, the `Connection_cache.call` function no longer blocks on connection establishment but rather awaits the connection promise.