In this change list
1. validate HLL column when loading data, if data is invalid, this row
will be filtered.
2. seems as empty HLL when serializing invalid type of HLL data, with
this change, all ingested data will be valid.
3. seems as empty HLL when deserializing nullptr or invalid type of HLL data.
With this change, dirty data can be handled normally.
4. rename function empty_hll to hll_empty.
5. disable memtable_flush_execute_test because this will fails
sometimes. When tearing down, some thread is not joined, and they will
visit destroyed resource, which is invalid.
