RFC: support pass unique key to Keeper about fastflow

[ShiningRush]

Hi echo, thanks for your proposal, basically I agree with it.
I want declare two points:

1. Worker key limited 0~255 is incorrect, sonyflake is support to use uint16(0~65536) as machine_id, this should be a bug of fastflow
2. using worker key suffix as unique number is very convenient in k8s, because you can deploy your application by stateful set, it can auto generate pod name like stateful-set-0, stateful-set-1 ...

however, support a new way to generate id is LGTM. I think we can decouple id generator:

• if client give a id generator, we could do not check the format of worker key any more
• or else, nothing change