Liveness/readiness checks

[kvosper]

Adds endpoints for checking:

1. Styx is live (i.e. running)
2. Styx is ready (to serve traffic)

[bestokes]

@mikkokar please can you review?

[mikkokar]

I propose an alternative approach for the readiness checks: Introduce a StaticResponseHadler in the Styx proxy pipeline to serve the readiness responses. Then tag all readiness requests with a specific X-Styx-Readiness-Check header and divert them to the StaticResponseHandler. Let all other requests to pass through.

Then you could do all the readiness checks directly to Styx HTTP and HTTPS ports.

Something to think about.

As per Mikko's review we are changing the approach.

[mikkokar]

Some observations:

• Most of our toString methods conform to AbstractStyxService{name=<servicename>} format. Worth considering here?

• Also print service status?

• A service is a stateful object and styx can have two services with identical names. To distinguish between two instances it might be necessary to include object identity in the output.

[mikkokar]

Please provide an admin page link.

[mikkokar]

Why it is necessary to an HttpServer instance and wrap it into a ServerReference object?

Why not to create an HttpServer instance and pass it as is to ServerService? Like so:

new ServerService(createAdminServer(components));

[kvosper]

The whole point of the ServerService is to run the creation/start-up process of the server. If you do it before construction, then the service does nothing, and we would not be able to start the servers asynchronously.
The ServerReference is because the StyxServer needs to receive the server after it has been created asynchronously.

[mikkokar]

createAdminServer() just creates a server object. It doesn't start it. The service is still be able to start it by invoking startAsync.

Okay. I see. The createProxyServer blocks until the plugins are loaded. This is somewhat confusing:

createProxyServer() returns an HttpServer instance which has startAsync method. Typical usage is:

HttpServer server = createProxyServer();
server.startAsync().awaitRunning();

Where does it block? On both lines. Surprisingly. The first line blocks at loading the plugins, the 2nd line blocks at waiting for the server to start.

IMO it would be more sensible to wait for plugins in the service startup method, rather than in HttpServer creation.

[kvosper]

We have decided not to go ahead with this PR, however it did spawn two smaller pieces of work:

#389
#390