[FEATURE] add more useful worker classes

[kemingy]

So far, there is only one Worker base type with JSON serialization and Pickle IPC serialization.

• [ ] #139
• [x] #140

[gaocegege]

I think we can add some docs for the worker types. I did not get the differences between JSON and Pickle before we discussed them offline.

[kemingy]

I think we can add some docs for the worker types. I did not get the differences between JSON and Pickle before we discussed them offline.

Yeah. Although we have the doc, but it's not straightforward. I'm planning to draw a flowchart to demonstrate the main features.

[kemingy]

BTW, it should be a mixin. Check https://stackoverflow.com/a/547714

[lkevinzc]

BTW, it should be a mixin. Check https://stackoverflow.com/a/547714

Yea agree, this looks better.