App lifecycle handler

[mfranciszkiewicz]

The main responsibility of the AppLifecycleHandler class is to enable custom code execution before/after AppServer._server startup/shutdown. This pins the potential points of execution of the following:

• app bootstrapping
• app clean up
• starting up and tearing down background jobs

The AppLifecycleHandler is currently a stub of a planned default implementation responsible for thread management (indirectly).

Additionally, AppLifecycleHandler objects:

• own the shutdown_future
• expose a request_shutdown method, resolving the aforementioned future

[Wiezzel]

@Krigpl Any comments? Can we merge this?

[Krigpl]

I just wonder whether having a separate class here instead of providing empty methods directly in the handler has any added value. I imagine that usually some resources will be used both by the main handler and the lifecycle handler and things may get awkward. But otherwise ok.

[Wiezzel]

@mfranciszkiewicz Plz bump to v0.15.0 because I already released 0.14.0 with the shutdown change.