Clean-up transaction begin

Move it into a dedicated function instead of being part
of the `Transaction` constructor.

Author: lutovich

src/v1/session.js

@@ -110,8 +110,13 @@ class Session {
     connectionHolder.initializeConnection();
     this._hasTx = true;
 
-    const onTxClose = () => this._hasTx = false;
-    return new Transaction(connectionHolder, onTxClose.bind(this), this._lastBookmark, this._updateBookmark.bind(this));
+    const tx = new Transaction(connectionHolder, this._transactionClosed.bind(this), this._updateBookmark.bind(this));
+    tx._begin(this._lastBookmark);
+    return tx;
+  }
+
+  _transactionClosed() {
+    this._hasTx = false;
   }
 
   /**

src/v1/transaction.js

@@ -32,20 +32,21 @@ class Transaction {
    * @constructor
    * @param {ConnectionHolder} connectionHolder - the connection holder to get connection from.
    * @param {function()} onClose - Function to be called when transaction is committed or rolled back.
-   * @param {Bookmark} bookmark bookmark for transaction begin.
    * @param {function(bookmark: Bookmark)} onBookmark callback invoked when new bookmark is produced.
    */
-  constructor(connectionHolder, onClose, bookmark, onBookmark) {
+  constructor(connectionHolder, onClose, onBookmark) {
     this._connectionHolder = connectionHolder;
+    this._state = _states.ACTIVE;
+    this._onClose = onClose;
+    this._onBookmark = onBookmark;
+  }
+
+  _begin(bookmark) {
     const streamObserver = new _TransactionStreamObserver(this);
 
     this._connectionHolder.getConnection(streamObserver)
       .then(conn => conn.protocol().beginTransaction(bookmark, streamObserver))
       .catch(error => streamObserver.onError(error));
-
-    this._state = _states.ACTIVE;
-    this._onClose = onClose;
-    this._onBookmark = onBookmark;
   }
 
   /**