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/kvstore.go

@@ -0,0 +1,44 @@
+package engine
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+
+	"github.com/rogpeppe/go-internal/lockedfile"
+)
+
+// KVStore is a simple, atomic key-value store. The user of
+// probe-engine should supply an implementation of this interface,
+// which will be used by probe-engine to store specific data.
+type KVStore interface {
+	Get(key string) (value []byte, err error)
+	Set(key string, value []byte) (err error)
+}
+
+// FileSystemKVStore is a directory based KVStore
+type FileSystemKVStore struct {
+	basedir string
+}
+
+// NewFileSystemKVStore creates a new FileSystemKVStore.
+func NewFileSystemKVStore(basedir string) (kvs *FileSystemKVStore, err error) {
+	if err = os.MkdirAll(basedir, 0700); err == nil {
+		kvs = &FileSystemKVStore{basedir: basedir}
+	}
+	return
+}
+
+func (kvs *FileSystemKVStore) filename(key string) string {
+	return filepath.Join(kvs.basedir, key)
+}
+
+// Get returns the specified key's value
+func (kvs *FileSystemKVStore) Get(key string) ([]byte, error) {
+	return lockedfile.Read(kvs.filename(key))
+}
+
+// Set sets the value of a specific key
+func (kvs *FileSystemKVStore) Set(key string, value []byte) error {
+	return lockedfile.Write(kvs.filename(key), bytes.NewReader(value), 0600)
+}