Add a builder for arbiter + alive method

[Sytten]

PR Type

Feature

PR Checklist

Check your PR fulfills the following:

• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] A changelog entry has been made for the appropriate packages.
• [x] Format code with the latest stable rustfmt

Overview

Builder

This adds a builder for the arbiter, allowing more customization of the thread used by the arbiter.

The first customizable I added was the name, this allows us to differentiate the different arbiters better since they are not all used for the same purpose.

Alive

I also added an alive method on the arbiter so the caller can try to detect dead arbiters before sending it a future since it consumes it. Otherwise you need to have a clone future which is not good most of the time.

[robjtede]

LGTM from a quick look but needs a manual rebase (which I don't have perms to do on this PR).