chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/experiment/ndt7/upload.go

@@ -0,0 +1,75 @@
+package ndt7
+
+import (
+	"context"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+func newMessage(n int) (*websocket.PreparedMessage, error) {
+	return websocket.NewPreparedMessage(websocket.BinaryMessage, make([]byte, n))
+}
+
+type uploadManager struct {
+	conn                 mockableConn
+	fractionForScaling   int64
+	maxRuntime           time.Duration
+	maxMessageSize       int
+	maxScaledMessageSize int
+	measureInterval      time.Duration
+	minMessageSize       int
+	newMessage           func(int) (*websocket.PreparedMessage, error)
+	onPerformance        callbackPerformance
+}
+
+func newUploadManager(
+	conn mockableConn, onPerformance callbackPerformance,
+) uploadManager {
+	return uploadManager{
+		conn:                 conn,
+		fractionForScaling:   paramFractionForScaling,
+		maxRuntime:           paramMaxRuntime,
+		maxMessageSize:       paramMaxMessageSize,
+		maxScaledMessageSize: paramMaxScaledMessageSize,
+		measureInterval:      paramMeasureInterval,
+		minMessageSize:       paramMinMessageSize,
+		newMessage:           newMessage,
+		onPerformance:        onPerformance,
+	}
+}
+
+func (mgr uploadManager) run(ctx context.Context) error {
+	var total int64
+	start := time.Now()
+	if err := mgr.conn.SetWriteDeadline(time.Now().Add(mgr.maxRuntime)); err != nil {
+		return err
+	}
+	size := mgr.minMessageSize
+	message, err := mgr.newMessage(size)
+	if err != nil {
+		return err
+	}
+	ticker := time.NewTicker(mgr.measureInterval)
+	defer ticker.Stop()
+	for ctx.Err() == nil {
+		if err := mgr.conn.WritePreparedMessage(message); err != nil {
+			return err
+		}
+		total += int64(size)
+		select {
+		case now := <-ticker.C:
+			mgr.onPerformance(now.Sub(start), total)
+		default:
+			// NOTHING
+		}
+		if size >= mgr.maxScaledMessageSize || int64(size) >= (total/mgr.fractionForScaling) {
+			continue
+		}
+		size <<= 1
+		if message, err = mgr.newMessage(size); err != nil {
+			return err
+		}
+	}
+	return nil
+}