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/context.go

@@ -1,5 +1,9 @@
 package bytecounter
 
+//
+// Implicit byte counting based on context
+//
+
 import (
 	"context"
 	"net"