Multiple worker groups support

[wenyuzhao]

We only have one worker group hard-coded in the work packet scheduler.

However, there are three use cases for the multiple worker groups generalization:

1. LXR requires another worker group with a different number of workers for concurrent GC.
2. Single-threaded work packet Single-threaded work packets #207
3. @tianleq 's thread-local GC also requires multiple worker groups.