refactor: handle timeout in the connection generator, not the waiting function

refactor: handle timeout in the connection generator, not the waiting function

The waiting function is supposed to be generic, the timeout is a policy
decision of the generator.

This change aligns the semantics of the `timeout` parameter of `wait_conn()`
to the one of the other wait functions: the timeout is actually an
interval. It will be renamed to clarify that in a followup commit.