NATS no responders support

[jirenius]

Issue

NATS server v2.2 introduced headers that can be used to post meta-data. More specifically, when a request is sent to NATS on a subject which no service is listening on, it can instantly send back a response with the Status header set to 503, indicating that the request has no responders.

Resgate should support dealing with Status 503 headers, turning them into system.notFound error responses.

Note

• system.timeout would only be sent by Resgate if a service actually receives the request without responding in time.
• system.notFound will be sent directly instead of the current 3 second delay waiting for the timeout.

[jirenius]

Resolved in PR #224