Add on_shutdown with options abandon and release

[yaauie]

In some configurations, it would be helpful for the input plugin's consumers to unsubscribe from their topics during shutdown, especially with sticky-type partition assignment strategies with non-static group membership.

This would allow the consumer's assigned partitions to be immediately eligible to be assigned to another member of the group, instead of needing to wait out broker-side timeouts.

This patch introduces a new option on_shutdown, with two options:

• release: revoke all current subscriptions, releasing the assigned partitions so that they can be immediately assigned to another member of the group
• abandon (backward-compatible default): leave the subscription in-tact when closing, allowing broker to begin reassigning these partitions in its own time