Add #tryLocalTx, #eitherLocalTx to be consistent with #futureLocalTx

refs #300 
```scala
def eitherLocalTx[A, B](execution: DBSession => Either[B, A])(implicit context: CPContext = NoCPContext): Either[B, A]
```

or

```scala
def eitherLocalTx[A, B <: Throwable](execution: DBSession => Either[B, A])(implicit context: CPContext = NoCPContext): Either[B, A]
```

?