global knob to reduce the number of domains in domain pool

[c-cube]

see https://github.com/c-cube/moonpool/issues/41#issuecomment-3517156401 : we might still have to share domains with other libraries, or be on a system where Domain.recommended_domain_count() is just wrong. The only way to deal with that is to change the domain_pool size early in the process' lifetime.