Refactor Target.java and add tons of tests

Make Target an interface implemented by SrvTarget and ExplicitTarget.
This allows us to not collapse multiple master endpoints into one
target, which will enable better CLI output later on.

E.g. `helios jobs --master master1 --master master2 --json` outputs

```
{
  master1: {A},
  master2: {B}
}
```

instead of

```
{
  A+B
}
```
Like the `ExplicitTarget.from(...)`, probably better to have this return `List<SrvTarget>`. It's a little odd to have `SrvTarget.from(...)` return something other than `SrvTarget`. Callers who need a List of `Target` will need to do something like `List<? extends Target> list = SrvTarget.from(...)`