Consider providing a non-concurrent Saga

[LukaJCB]

Consider providing a non-concurrent Saga

[VladKopanev]

I think you are right. I also thought that this Concurrent requirement is a design flaw, but the solution was not obvious for me at that time. Thanks for this idea, I think we should try to implement it!

[benhutchison]

This is biting me because I also use MTL MonadState in the F stack, and there's no StateT instance for Concurrent. The lack of StateT+Concurrent is, IIUC, because if the Concurrent forks Fibers, state updates would have to be discarded on all but one of the branches.

[VladKopanev]

@benhutchison could you please look at my PR #51 ? It moves Concurrent typeclass requirement to Saga methods that actually meant to do parallel composition. So transact requires MonadError[F, Throwable] now.