Allow to start multi schedulers

[xzander]

Here is PR to lock for multi schedulers without register birth and death.
When first scheduler starts, another wait for lock. While first one is working it prolongs lock's expire time more then _interval. So second scheduler is allowed to get lock only when first one is dead.

[selwin]

Having the ability for multiple schedulers to run simultaneously is something I want to have. However, each scheduler still needs to register their births and deaths so we can keep track of active schedulers (similar to how RQ does it) https://github.com/rq/rq/blob/master/rq/worker.py#L272

Could you please update the PR accordingly?

[selwin]

Fixed in #212