Add command for warming cache

It may make sense to have a command to warm the cache. A user could warm the cache by scannning the table. However this has to deserialize all of the key values. A specialized command for warming the cache could just read blocks from files into the cache without deserializing anything, which would be much faster.

Could possibly have a shell command like the following.

```
user@instance> warm -t -b -e
```

Start by clarifying the proposed warm command's scope, including table and row-range handling. The issue describes reading blocks into the cache without deserializing key values; done requires an agreed command interface and behavior, followed by implementation and tests.

Written by the indexing model from the issue text.