Support Graceful shutdown

[skovtunenko]

Hello, in version v2.2.1, there is no support for graceful shutdown for the application.

The application and its jobs needs to know (and react accordingly) when there is a time to stop executing the flow.

I assume this is somewhat related to #17

[fieldryand]

Great point. Starting with v3.0.1 (still in pre-release) the application accepts a context allowing graceful shutdown, so that running jobs that get interrupted will be marked as failed. The context is also passed to the task operator, allowing users to implement any clean-up actions for interrupted tasks.