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
Like we did before for the resolver, a dialer should propagate the
request to close idle connections to underlying types.
See https://github.com/ooni/probe/issues/1591
Auto-configure every relevant TLS field as close as possible to
where it's actually used.
As a side effect, add support for mocking the creation of a TLS
connection, which should possibly be useful for uTLS?
Work that is part of https://github.com/ooni/probe/issues/1505
