Rename service.Application.SignalTestComplete to Shutdown

[rakyll]

SignalTestComplete doesn't belong to an external API, rename this method to Shutdown and follow up with #483 in the future.

[rakyll]

Alternative, document that SignalTestComplete API is not stable and might be removed in the future.

[tigrannajaryan]

+1. We should rename to Shutdown and also rename stopTestChan to just stopChan. I can't see anything special about this stopping that is only applicable to tests. It is just a way to stop the service.

[tigrannajaryan]

@rakyll would you like to submit a PR that fixes this?

[rakyll]

Surely.