Feat: some functions in disktable need to be implemented

These functions are already implemented in memtable and tested in table_test. They should be implemented in disktable. After implementing them, tests in table_test need to be updated.
``` cpp
bool DiskTable::DeleteIndex(const std::string& idx_name)
uint64_t DiskTable::GetRecordIdxCnt()
bool DiskTable::GetRecordIdxCnt(uint32_t idx, uint64_t** stat, uint32_t* size)
uint64_t DiskTable::GetRecordPkCnt()
uint64_t DiskTable::GetRecordIdxByteSize() {
uint64_t DiskTable::Release()
```