v1.6.0

@selwin

Workers now listen to external commands via pubsub. The first two features taking advantage of this infrastructure are send_shutdown_command() and send_kill_horse_command(). Thanks @selwin!
Added job.last_heartbeat property that's periodically updated when job is running. Thanks @theambient!
Now horses are killed by their parent group. This helps in cleanly killing all related processes if job uses multiprocessing. Thanks @theambient!
Fixed scheduler usage with Redis connections that uses custom parser classes. Thanks @selwin!
Scheduler now enqueue jobs in batches to prevent lock timeouts. Thanks @nikkonrom!
Scheduler now follows RQ worker's logging configuration. Thanks @christopher-dG!

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