refactor(netxlite): add more functions to resolver (#455)

We would like to refactor the code so that a DoH resolver owns the
connections of its underlying HTTP client.

To do that, we need first to incorporate CloseIdleConnections
into the Resolver model. Then, we need to add the same function
to all netxlite types that wrap a Resolver type.

At the same time, we want the rest of the code for now to continue
with the simpler definition of a Resolver, now called ResolverLegacy.

We will eventually propagate this change to the rest of the tree
and simplify the way in which we manage Resolvers.

To make this possible, we introduce a new factory function that
adapts a ResolverLegacy to become a Resolver.

See https://github.com/ooni/probe/issues/1591.

internal/engine/netx/dialer/dialer.go

@@ -80,7 +80,10 @@ func New(config *Config, resolver Resolver) Dialer {
 	if config.ReadWriteSaver != nil {
 		d = &saverConnDialer{Dialer: d, Saver: config.ReadWriteSaver}
 	}
-	d = &netxlite.DialerResolver{Resolver: resolver, Dialer: d}
+	d = &netxlite.DialerResolver{
+		Resolver: netxlite.NewResolverLegacyAdapter(resolver),
+		Dialer:   d,
+	}
 	d = &proxyDialer{ProxyURL: config.ProxyURL, Dialer: d}
 	if config.ContextByteCounting {
 		d = &byteCounterDialer{Dialer: d}