Sayonara, ReactiveUI

This PR breaks the dependency between ReactiveUI and Akavache, via using the new features in Splat (paulcbetts/splat#32 and paulcbetts/splat#33). 

### What does this do?

* Splat doesn't depend on Rx, so one thing that was actually tricky was to deal with `RxApp.TaskpoolScheduler` - instead, we define our own, with a planned "secret handshake" with RxUI 6.0 which will register using the locator the fallback that Akavache looks for
* Getting the correct file paths automatically now doesn't require Akavache.Mobile, which was always kind of dumb (originally it was done so that Akavache core was Linux compatible, but those folks are on their own)

### Breaking Changes

* If you expected things to become synchronous in the test runner when using Akavache, it might not now unless you're also using RxUI 6.0. Probably nobody cares though about this

### TODO:

- [ ] Wait until those Splat PRs are merged