Wrong result for MemTableTraverseIterator::GetCount()

When using traverseIterator to traverse Memtable, GetCount() is used to get the traverse count. The meaning of 'traverse count' is not clear. and if we regard it as 'how many pieces of data it has traversed' then the result is not correct.

## Situation
``` cpp
TEST_F(TableTest, TraverseIteratorCount) {
::openmldb::api::TableMeta table_meta;
table_meta.set_name("table1");
table_meta.set_tid(1);
table_meta.set_pid(0);
table_meta.set_seg_cnt(8);
table_meta.set_mode(::openmldb::api::TableMode::kTableLeader);
table_meta.set_key_entry_max_height(8);
table_meta.set_format_version(1);
SchemaCodec::SetColumnDesc(table_meta.add_column_desc(), "card", ::openmldb::type::kString);
SchemaCodec::SetColumnDesc(table_meta.add_column_desc(), "mcc", ::openmldb::type::kString);
SchemaCodec::SetColumnDesc(table_meta.add_column_desc(), "price", ::openmldb::type::kBigInt);
SchemaCodec::SetColumnDesc(table_meta.add_column_desc(), "ts1", ::openmldb::type::kBigInt);
SchemaCodec::SetColumnDesc(table_meta.add_column_desc(), "ts2", ::openmldb::type::kBigInt);
SchemaCodec::SetIndex(table_meta.add_column_key(), "card", "card", "ts1", ::openmldb::type::kAbsoluteTime, 0, 0);
SchemaCodec::SetIndex(table_meta.add_column_key(), "card1", "card", "ts2", ::openmldb::type::kAbsoluteTime, 0, 0);
SchemaCodec::SetIndex(table_meta.add_column_key(), "mcc", "mcc", "ts1", ::openmldb::type::kAbsoluteTime, 0, 0);

MemTable table(table_meta);
table.Init();
codec::SDKCodec codec(table_meta);

for (int i = 0; i < 1000; i++) {
std::vector row = {"card" + std::to_string(i % 100), "mcc" + std::to_string(i),
"13", std::to_string(1000 + i), std::to_string(10000 + i)};
::openmldb::api::PutRequest request;
::openmldb::api::Dimension* dim = request.add_dimensions();
dim->set_idx(0);
dim->set_key(row[0]);
dim = request.add_dimensions();
dim->set_idx(1);
dim->set_key(row[0]);
dim = request.add_dimensions();
dim->set_idx(2);
dim->set_key(row[1]);
std::string value;
ASSERT_EQ(0, codec.EncodeRow(row, &value));
table.Put(0, value, request.dimensions());
}
TableIterator* it = table.NewTraverseIterator(0);
it->SeekToFirst();
int count = 0;
while (it->Valid()) {
count++;
it->Next();
}
ASSERT_EQ(1000, count);
ASSERT_EQ(1100, (int64_t)it->GetCount());
delete it;
}
```

## Expected Behavior

The result of (int64_t)it->GetCount() should probably be 1000

## Current Behavior

It's 1100 as the test says.

## Steps to Reproduce
make and run table_test, in TEST_F(TableTest, TraverseIteratorCount)