Rename num_domains to num_additional_domains in setup_pool

[Sudha247]

Mentions that we actually create a pool of num_domains + 1 along with the parent domain.

[kayceesrk]

Would it be useful to change the num_domains parameter as num_additional_domains to make it explicit? This is a breaking change. @ctk21 what are your thoughts?

[ctk21]

I can see a clarity advantage to cleaning that up to num_additional_domains (or num_workers) because I can see it being confusing as it is.

We will probably need to make a release of domainslib soon that switches us from using critical_section to Mutex/Condition, so getting the change in for that release might make sense.

[ctk21]

Looks good to me.