[#1255] node/session: Create separate dir for in-memory storage

Move in-memory session storage to the separate directory of `storage`. It is
done for future support of different kind of session storages.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/services/session/storage/temporary/storage.go

@@ -0,0 +1,65 @@
+package temporary
+
+import (
+	"sync"
+
+	"github.com/mr-tron/base58"
+	"github.com/nspcc-dev/neofs-node/pkg/services/session/storage"
+	"github.com/nspcc-dev/neofs-sdk-go/owner"
+)
+
+type key struct {
+	tokenID string
+	ownerID string
+}
+
+// TokenStore is an in-memory session token store.
+// It allows creating (storing), retrieving and
+// expiring (removing) session tokens.
+// Must be created only via calling NewTokenStore.
+type TokenStore struct {
+	mtx *sync.RWMutex
+
+	tokens map[key]*storage.PrivateToken
+}
+
+// NewTokenStore creates, initializes and returns a new TokenStore instance.
+//
+// The elements of the instance are stored in the map.
+func NewTokenStore() *TokenStore {
+	return &TokenStore{
+		mtx:    new(sync.RWMutex),
+		tokens: make(map[key]*storage.PrivateToken),
+	}
+}
+
+// Get returns private token corresponding to the given identifiers.
+//
+// Returns nil is there is no element in storage.
+func (s *TokenStore) Get(ownerID *owner.ID, tokenID []byte) *storage.PrivateToken {
+	ownerBytes, err := ownerID.Marshal()
+	if err != nil {
+		panic(err)
+	}
+
+	s.mtx.RLock()
+	t := s.tokens[key{
+		tokenID: base58.Encode(tokenID),
+		ownerID: base58.Encode(ownerBytes),
+	}]
+	s.mtx.RUnlock()
+
+	return t
+}
+
+// RemoveOld removes all tokens expired since provided epoch.
+func (s *TokenStore) RemoveOld(epoch uint64) {
+	s.mtx.Lock()
+	defer s.mtx.Unlock()
+
+	for k, tok := range s.tokens {
+		if tok.ExpiredAt() <= epoch {
+			delete(s.tokens, k)
+		}
+	}
+}