[YSQL] Update with expression pushdown may cause data inconsistency

[d-uspenskiy]

Description

The following test fails on latest master

TEST_F(PgMiniTest, YB_DISABLE_TEST_IN_TSAN(ConcurrentSingleRowUpdate)) {
  auto conn = ASSERT_RESULT(Connect());
  ASSERT_OK(conn.Execute("CREATE TABLE t(k INT PRIMARY KEY, counter INT)"));
  ASSERT_OK(conn.Execute("INSERT INTO t VALUES(1, 0)"));
  const size_t thread_count = 10;
  const size_t increment_per_thread = 5;
  {
    CountDownLatch latch(thread_count);
    TestThreadHolder thread_holder;
    for (size_t i = 0; i < thread_count; ++i) {
      thread_holder.AddThreadFunctor([this, &stop = thread_holder.stop_flag(), &latch] {
        auto thread_conn = ASSERT_RESULT(Connect());
        ASSERT_OK(thread_conn.Execute("SET yb_enable_expression_pushdown TO true"));
        latch.CountDown();
        latch.Wait();
        for (size_t j = 0; j < increment_per_thread; ++j) {
          ASSERT_OK(thread_conn.Execute("UPDATE t SET counter = counter + 1 WHERE k = 1"));
        }
      });
    }
  }
  auto res = ASSERT_RESULT(conn.Fetch("SELECT counter FROM t WHERE k = 1"));
  ASSERT_EQ(1, PQnfields(res.get()));
  ASSERT_EQ(1, PQntuples(res.get()));
  auto counter = ASSERT_RESULT(GetInt32(res.get(), 0, 0));
  ASSERT_EQ(thread_count * increment_per_thread, counter);
}

Reason: Non transactional write operation uses read-time as a result this write operation may see non latest state of the DocDB.