refactor(session.go): replace engine/netx with netxlite (#767)

This diff replaces engine/netx code with netxlite code in
the engine/session.go file. To this end, we needed to move
some code from engine/netx to netxlite. While there, we
did review and improve the unit tests.

A notable change in this diff is (or seems to be) that in
engine/session.go we're not filtering for bogons anymore so
that, in principle, we could believe a resolver returning
to us bogon IP addresses for OONI services. However, I did
not bother with changing bogons filtering because the
sessionresolver package is already filtering for bogons,
so it is actually okay to avoid doing that again the
session.go code. See:

https://github.com/ooni/probe-cli/blob/v3.15.0-alpha.1/internal/engine/internal/sessionresolver/resolvermaker.go#L88

There are two reference issues for this cleanup:

1. https://github.com/ooni/probe/issues/2115

2. https://github.com/ooni/probe/issues/2121

internal/engine/netx/netx.go

@@ -202,7 +202,7 @@ func NewHTTPTransport(config Config) model.HTTPTransport {
 		TLSConfig: config.TLSConfig})
 
 	if config.ByteCounter != nil {
-		txp = httptransport.ByteCountingTransport{
+		txp = &httptransport.ByteCountingTransport{
 			Counter: config.ByteCounter, HTTPTransport: txp}
 	}
 	if config.Logger != nil {