Requests more messages from the stream. We disable the auto flow control mechanisam in grpc,
so we need to request messages ourself. This gives us more control over how much buffer we
maintain in the client. Grpc will request 1 initial message automatically so we don't need to
call this at the beginning. After that it should be called whenever there is a flow control
window available based on the flow control setting configured by the client. Currently we do
not have any flow control so this is called automatically when a message is received.