Ensure multiple subscriptions can be handled about eventstore-client-go

[pvanbuijtene]

There's a catch on reading and writing to streams, as mentioned here:

Thread-safety: note that client-side RPC invocations and server-side RPC handlers are thread-safe and are meant to be run on concurrent goroutines. But also note that for individual streams, incoming and outgoing data is bi-directional but serial; so e.g. individual streams do not support concurrent reads or concurrent writes (but reads are safely concurrent with writes).

[YoEight]

Fixed by #56