merge the two versions of ScanIndex in tablet_impl.cc

[zhanghaohit]

Describe the feature you'd like

Most of the logics of the two ScanIndex are the same, except the output buffer. We'd better merge these two.

int32_t TabletImpl::ScanIndex(const ::openmldb::api::ScanRequest* request, const ::openmldb::api::TableMeta& meta,
                              const std::map<int32_t, std::shared_ptr<Schema>>& vers_schema,
                              CombineIterator* combine_it, std::string* pairs, uint32_t* count, bool* is_finish);
int32_t TabletImpl::ScanIndex(const ::openmldb::api::ScanRequest* request, const ::openmldb::api::TableMeta& meta,
                              const std::map<int32_t, std::shared_ptr<Schema>>& vers_schema,
                              CombineIterator* combine_it, butil::IOBuf* io_buf, uint32_t* count, bool* is_finish);

[jkpjkpjkp]

Though 2 ScanIndex functions have 40ish identical lines, I see no obvious way to elegantly merge them together, lest using a global define, since their common lines are mainly declarations of local variables.

[try-agaaain]

I am interested in this task, can it be assigned to me?

[zhanghaohit]

I am interested in this task, can it be assigned to me?

Thanks for your interest. I've assigned this task to you.

[try-agaaain]

In order to merge these two ScanIndex functions, I have set up the compilation environment and compiled tablet_impl_test for debugging. However, because I do not know the business logic, I cannot clearly understand the implementation process of the two ScanIndex functions. Could you please give me some suggestions?

[dl239]

another function deleted in #3572