[Ray Core] Actor On Finish Function

[karansampath]

Description

It would be useful to specify actions to be taken on finishing an actor's process, a finish function that acts as the companion to the __init__ function. This would allow for specification of 'how to die', which would be immensely useful.

Use case

This feature would allow actors to gracefully end, so that any remaining data and information contained within it is not lost and can be used to start off processes afterward, therefore allowing for more complex orchestration.