(feat) Ability to supply your own shared tokio runtime

[john-sharratt]

Modified the shared tokio runtime so that they can be set before they are lazy loaded thus allowing the caller to pass in their own runtime

[john-sharratt]

@syrusakbary @theduke lets try and close this pull request out please - its needed to fix a stability issue for concurrent runtimes that fight each other

[theduke]

The downside to this is that it will always require a separate runtime now.

Most users would just invoke this from an async context without knowing the internals, and thus be forced into always having two separate tokio runtimes active at once.

I guess it's still a reasonable tradeoff to make, though I do think that specifying the runtime should just be mandatory.

[theduke]

Let's merge this for now regardless.