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/internal/kvstore/kvstore.go

@@ -0,0 +1,44 @@
+// Package kvstore contains key-value stores
+package kvstore
+
+import (
+	"errors"
+	"sync"
+)
+
+// MemoryKeyValueStore is an in-memory key-value store
+type MemoryKeyValueStore struct {
+	m  map[string][]byte
+	mu sync.Mutex
+}
+
+// NewMemoryKeyValueStore creates a new in-memory key-value store
+func NewMemoryKeyValueStore() *MemoryKeyValueStore {
+	return &MemoryKeyValueStore{
+		m: make(map[string][]byte),
+	}
+}
+
+// Get returns a key from the key value store
+func (kvs *MemoryKeyValueStore) Get(key string) ([]byte, error) {
+	var (
+		err   error
+		ok    bool
+		value []byte
+	)
+	kvs.mu.Lock()
+	defer kvs.mu.Unlock()
+	value, ok = kvs.m[key]
+	if !ok {
+		err = errors.New("no such key")
+	}
+	return value, err
+}
+
+// Set sets a key into the key value store
+func (kvs *MemoryKeyValueStore) Set(key string, value []byte) error {
+	kvs.mu.Lock()
+	defer kvs.mu.Unlock()
+	kvs.m[key] = value
+	return nil
+}