refactor(netxlite): adapt single-use-quic-dialer from websteps (#472)

This is the last bit of functionality we need before rewriting a
chunk of websteps to use netxlite.

To rewrite most of it, we still need to move over:

1. dnstransport code

2. errorsx code

With both done, netxlite is a good library for websteps as well
as for most other operations we perform outside of the experiments.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dialer.go

@@ -164,7 +164,7 @@ func NewSingleUseDialer(conn net.Conn) Dialer {
 	return &dialerSingleUse{conn: conn}
 }
 
-// dialerSingleUse is the type of Dialer returned by NewSingleDialer.
+// dialerSingleUse is the Dialer returned by NewSingleDialer.
 type dialerSingleUse struct {
 	sync.Mutex
 	conn net.Conn