Add a "modified" timestamp for key-value pairs

It would be great if key-value pairs had a timestamp of when they were last modified (or created).
For example:
```
var box = Hive.box('myBox');
box.put('name', 'David');
DateTime modified = box.modified('name');
```

Start by tracing the Hive.box and box.put entry points shown in the example to understand where key-value metadata is stored. Define how box.modified('name') should behave for created and updated pairs, then verify that it returns the expected DateTime after writes.

Written by the indexing model from the issue text.