MetadataSchema convenience Text methods for constants does not enforce strict immutability

The convenience definitions that provide a `Text` object for constants in MetadataSchema expose the constants to changes if the Text object is mutated. For example:

```
public static class ServerColumnFamily {
public static final String STR_NAME = "srv";
public static final Text NAME = new Text(STR_NAME);
...
```

Anything that uses `ServerColumnFamily.NAME` and then writes to that object will change the definition for everything using that `ServerColumnFamily.NAME` object,

Options:

- If it is not a significant impact, could those convenience definitions of Text objects be removed?
- Subclass the Text object in these cases to enforce immutability?
- Use a immutable "byte-array" class instead of Text?