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/download.go

@@ -0,0 +1,73 @@
+package ndt7
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+type downloadManager struct {
+	conn            mockableConn
+	maxMessageSize  int64
+	maxRuntime      time.Duration
+	measureInterval time.Duration
+	onJSON          callbackJSON
+	onPerformance   callbackPerformance
+}
+
+func newDownloadManager(
+	conn mockableConn, onPerformance callbackPerformance,
+	onJSON callbackJSON,
+) downloadManager {
+	return downloadManager{
+		conn:            conn,
+		maxMessageSize:  paramMaxMessageSize,
+		maxRuntime:      paramMaxRuntime,
+		measureInterval: paramMeasureInterval,
+		onJSON:          onJSON,
+		onPerformance:   onPerformance,
+	}
+}
+
+func (mgr downloadManager) run(ctx context.Context) error {
+	var total int64
+	start := time.Now()
+	if err := mgr.conn.SetReadDeadline(start.Add(mgr.maxRuntime)); err != nil {
+		return err
+	}
+	mgr.conn.SetReadLimit(mgr.maxMessageSize)
+	ticker := time.NewTicker(mgr.measureInterval)
+	defer ticker.Stop()
+	for ctx.Err() == nil {
+		kind, reader, err := mgr.conn.NextReader()
+		if err != nil {
+			return err
+		}
+		if kind == websocket.TextMessage {
+			data, err := ioutil.ReadAll(reader)
+			if err != nil {
+				return err
+			}
+			total += int64(len(data))
+			if err := mgr.onJSON(data); err != nil {
+				return err
+			}
+			continue
+		}
+		n, err := io.Copy(ioutil.Discard, reader)
+		if err != nil {
+			return err
+		}
+		total += int64(n)
+		select {
+		case now := <-ticker.C:
+			mgr.onPerformance(now.Sub(start), total)
+		default:
+			// NOTHING
+		}
+	}
+	return nil
+}