[WIP] Revert RACAction, use signal generators instead

**:warning: NOT READY TO MERGE :warning:**

`RACAction` doesn't make [pure FRP](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/992) any easier. Let's focus on the patterns that do.

Resolves #992 and #993.

**To do:**

 - [ ] Reintroduce a [transactional signal generator](https://github.com/ReactiveCocoa/ReactiveCocoa/pull/936) to recover `executing` and `executionSignals` behavior.
 - [ ] Update mentions of `RACAction` in documentation and deprecation messages.