refactor: move bytecounter to internal (#391)

It's generic enough to live outside of engine/netx.

Occurred to me while working on https://github.com/ooni/probe/issues/1687.

internal/engine/netx/bytecounter/bytecounter.go

@@ -1,54 +0,0 @@
-package bytecounter
-
-import "github.com/ooni/probe-cli/v3/internal/atomicx"
-
-// Counter counts bytes sent and received.
-type Counter struct {
-	Received *atomicx.Int64
-	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
-}