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/bytecounter/bytecounter.go

@@ -1,61 +0,0 @@
-// Package bytecounter contains code to track the number of
-// bytes sent and received by a probe.
-package bytecounter
-
-import "github.com/ooni/probe-cli/v3/internal/atomicx"
-
-// Counter counts bytes sent and received.
-type Counter struct {
-	// Received contains the bytes received. You MUST initialize
-	// this field, or you can just use the New factory.
-	Received *atomicx.Int64
-
-	// Sent contains the bytes sent. You MUST initialize
-	// this field, or you can just use the New factory.
-	Sent *atomicx.Int64
-}
-
-// New creates a new Counter.
-func New() *Counter {
-	return &Counter{Received: &atomicx.Int64{}, Sent: &atomicx.Int64{}}
-}
-
-// CountBytesSent adds count to the bytes sent counter.
-func (c *Counter) CountBytesSent(count int) {
-	c.Sent.Add(int64(count))
-}
-
-// CountKibiBytesSent adds 1024*count to the bytes sent counter.
-func (c *Counter) CountKibiBytesSent(count float64) {
-	c.Sent.Add(int64(1024 * count))
-}
-
-// BytesSent returns the bytes sent so far.
-func (c *Counter) BytesSent() int64 {
-	return c.Sent.Load()
-}
-
-// KibiBytesSent returns the KiB sent so far.
-func (c *Counter) KibiBytesSent() float64 {
-	return float64(c.BytesSent()) / 1024
-}
-
-// CountBytesReceived adds count to the bytes received counter.
-func (c *Counter) CountBytesReceived(count int) {
-	c.Received.Add(int64(count))
-}
-
-// CountKibiBytesReceived adds 1024*count to the bytes received counter.
-func (c *Counter) CountKibiBytesReceived(count float64) {
-	c.Received.Add(int64(1024 * count))
-}
-
-// BytesReceived returns the bytes received so far.
-func (c *Counter) BytesReceived() int64 {
-	return c.Received.Load()
-}
-
-// KibiBytesReceived returns the KiB received so far.
-func (c *Counter) KibiBytesReceived() float64 {
-	return float64(c.BytesReceived()) / 1024
-}