This basically adapts already existing code inside websteps to
instead be into the netxlite package, where it belongs.
In the process, abstract the TLSDialer but keep a reference to the
previous name to avoid refactoring existing code (just for now).
While there, notice that the right name is CloseIdleConnections (i.e.,
plural not singular) and change the name.
While there, since we abstracted TLSDialer to be an interface, create
suitable factories for making a TLSDialer type from a Dialer and a
TLSHandshaker.
See https://github.com/ooni/probe/issues/1591
2572376fdb