Data Race - Subscriptions not concurrent safe

[exapsy]

Error happens in

https://github.com/functionalfoundry/graphqlws/blob/master/subscriptions.go#L157

Seems like the subscription manager map is not concurrent safe.

[exapsy]

I fixed that by converting the map to sync.Map. Although, it required a little bit of an ugly solution as sync.Map doesn't support types. It's a map of interface{} which you have to dynamically check outside compile time if they're the right type. You can find the solution here

https://github.com/exapsy/graphqlws/blob/master/subscriptions.go

It actually solved a data-race problem of mine which crashed the entire frontend which was supported by the golang graphql server, which is nice.

But I think it's an ugly solution to serve to the public so I wont make a PR unless someone thinks it's alright. From what I see this repository's maintainers are inactive anyways so a PR probably wouldn't serve anyone :P

But whoever wants to utilize this solution into their code they're free to do so. :)