RFC: add createEpicEnhancer, deprecate createEpicMiddleware

There are currently two edge cases to Redux-Observable (RO) that don't have a clear an easy solution:

• Action order discrepancy
• Sync actions emitted at subscription are not seen by epics

While both of these issues can be worked around some Rx know-how and grit, it will catch seasoned people by surprise (I have been caught up by the second issue).

The major problem underlying both of these issues is the middleware API. It limits how we can intercept and dispatch actions going to and coming from epics. While there are possible solutions to both of these problems that would keep using the middleware API, they have their own set of tradeoffs and breaking changes.

I suggest we consider adding a createEpicEnhancer function that will create a store enhancer. This should give us the flexibility to cover these two issues (may help simplify SSR as well). At the same time we should deprecate createEpicMiddleware.

This would be a breaking change and should be thoroughly discussed by the community.