Fix race between jvm shutdown and `writer.finished`

[ttim]

Currently writer.finished happens in onComplete callback on Future result in Exucution. However since onComplete isn't being called before future being resolved and called asynchroniously after future being resolved, it leads to a race and runtime error:

• User's code as last operation in main executes Execution
• onComplete with writer.finished is being scheduled
• result Future gets resolved and jvm starts to shutdown
• writer.finished starts to execute and in case of cascading backend adds shutdown hook
• which is not permitted during jvm shutdown and breaks

To fix this behaviour I made onComplete logic to happen before result future get resolved by changing onComplete to andThen

[tlazaro]

LGTM, both onComplete vs andThen are for performing side-effects when the Future is done. But andThen makes it so the side-effect runs before anything else. Seems like exactly what we wanted in this case.

[ttim]

@johnynek does it seem reasonable to you? I'm wondering is there something I missing