[server] Allow setting Map function for resolving values in stream

[pekkah]

Instead of calling Executor.SubscribeAsync split the pipeline into two parts and use the lower level functions https://github.com/pekkah/fugu-graphql/blob/552d0aa6ac86cdea1aa1bdaad0e53ed473a6ec6c/src/graphql/execution/Subscription.cs#L97

https://github.com/pekkah/fugu-graphql/blob/552d0aa6ac86cdea1aa1bdaad0e53ed473a6ec6c/src/graphql/execution/Subscription.cs#L62

Allow overriding the default functions trough options.