Graceful shutdowns for blaze-server

[RaasAhsan]

I'm unhappy about the allocated call we're making for the Dispatcher, so we should probably gauge feasibility for getting graceful shutdowns.

[RaasAhsan]

So, I think this is going to be nontrivial at best. The problem is that the Dispatcher can be called on ExecutionContexts which are not necessarily bounded to the lifetime of the Dispatcher, which means that it can "go out of scope" and get leaked. This happens at least with IdleTimeoutStage and NIOHead1Stage.

Maybe we can add some kind of latch that shutdown can synchronize against which is released after there is no more selector activity

[yanns]

What do you think of http4s/http4s#4160 ? (only for the tests part)

[RaasAhsan]

@yanns I wasn't descriptive enough in the issue, I was actually referring to an allocated in BlazeServerBuilder, which is necessary for reasons listed above. The allocated in tests is less concerning but it's a good idea to do that too!