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
}
```
Since equals is defined, it's best practice to also define hashCode. IntelliJ can generate both for you.