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
}
```
I'd say it's customary to leave `equals` out of the interface. Just like `hashCode` and `toString`, these methods can be called on any object, so no need to put them in the interface. Others might disagree.