kRPC Websocket fallback

[rocketraman]

Is your feature request related to a problem? Please describe.
kRPC currently uses websockets.

While on the modern web websockets are generally reliable and usable, in some cases websockets may still not work due to infrastructure components such as older proxies or other similar limitations.

Describe the solution you'd like
It would be good for the kRPC transport to have a fallback in the case of a failure to establish a websocket. A long poll or SSE fallback for the KrpcTransport.receive method may be useful to increase resilience of kRPC in environments in which websockets are not possible to use.