Improve the experience of using stubs with streaming APIs

[sudorandom]

Currently fauxrpc will always emit a single message for server and bidi RPCs and then closes the request from the server. There should be a way to:

• continually emit messages until the client closes, with a configurable delay
• If the user specifies a sequence of stubs for a single "stub" entry then the sequence of messages will be sent before the server closes the request. This also needs to a configurable delay