Supporting run instance in specific worker pool

[sungjuly]

In my company cases, we need to run a specific instance(=workflow) on specific worker pool due to the input source location. I implemented that feature using modify in QueryAndOwnTransaction[1] protocol. I can contribute this feature if you guys want. Let me know if you guys want.

[1] https://github.com/pinterest/pinball/blob/master/pinball/master/transaction.py#L266

[csliu]

@sungjuly this seems cool, how about you submit your PR and then we can start discussing details about that.