go graphql/server: Close Mutation subscription after execution
go graphql/server: Close Mutation subscription after execution

Summary: Noticed this as a memory leak when we would upload a lot of
images through mutation endpoints.  The subscriptions for mutations are
never cleaned up, and the memory from the request (an image) would
remain in-memory until we'd kill the websocket connection.

samsarahq/thunder,samsarahq/thunder,samsarahq/thunder,samsarahq/thunder,samsarahq/thunder