Reset TransactionEngine internal state on Abort

Summary:
With neo4j java driver 1.7.0. they don't send `ROLLBACK`. This causes
unexpected nested transaction errors. This diff should fix that.

Reviewers: mferencevic

Reviewed By: mferencevic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1780

src/query/transaction_engine.hpp

@@ -93,6 +93,8 @@ class TransactionEngine final {
 
   void Abort() {
     results_ = std::experimental::nullopt;
+    expect_rollback_ = false;
+    in_explicit_transaction_ = false;
     if (!db_accessor_) return;
     db_accessor_->Abort();
     db_accessor_ = nullptr;