Subscription replay functionality

[athal7]

I've been working on enabling client functionality to specify a replayId when they subscribe, such that any messages that occurred during downtime can be delivered. There are a couple of places where I'm hooking into / reimplementing Absinthe functionality, and am interested in exploring if there is appetite for this functionality in the framework itself. Since I imagine there will be better insights from the authors on how to go about implementing it, here is a gist with the relevant pieces from my code, and we can figure out the best way to adapt it if desired.

[athal7]

If first-class support for replay isn't desired, something like #30 would make the implementation slightly less brittle in my specific application.

[athal7]

Just want to follow up on this that I now have this syncing across the cluster with mnesia using mnesiac and libcluster. @benwilson512 since this is something that could be generalized for any type of phoenix channel, I'll create a separate package for it and we can see if there's a good way to integrate after that

[athal7]

@benwilson512 https://github.com/athal7/dvr