rt: lazily detect number of CPUs (#2238)
rt: lazily detect number of CPUs (#2238)

This tweaks the runtime builder default to defer and lazily
auto-detect the number of CPUs. This is done in order to
avoid performing useless operations which may be expensive
on some platforms (e.g. Linux, where it is coupled to CPU
frequency probing).

Ref: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d5905dc14a87805a59f3c5bf70173aac2bb18f8
tokio-rs/tokio,tokio-rs/tokio