Some functions in tablet_impl and nameserver_impl only support memtable now

``` cpp
DeleteIndex
SendIndexData
LoadIndexData
ExtractIndexData
AddIndex
```

The issue mentions functions in tablet_impl and nameserver_impl that currently only support memtable. Look for these files in the codebase to understand the current implementations. Examine the listed functions (DeleteIndex, SendIndexData, LoadIndexData, ExtractIndexData, AddIndex) to see how they interact with memtable and what other storage engines might need support. Check for existing tests or documentation about index handling to understand the expected behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.