crimson/thread: pin thread pool to given CPU

crimson/thread: pin thread pool to given CPU

to take the full advantage of seastar reactor, we need to confine the
alien threads to given CPU core(s). in current setting, it's assumed
that alien threads do not perform CPU-bound tasks, so a single core
would suffice. we can always change the interface to pass a cpu_set_t
or a std::bitset if one CPU core is not enough.

Signed-off-by: Kefu Chai <kchai@redhat.com>