util: Use a mutex when retrieving threadpool data

util: Use a mutex when retrieving threadpool data

So far, the values the affected getters retrieve are static, i.e. there's no
way of changing them during runtime. But admin interface will later enable
not only getting but changing them as well. So to prevent phenomenons like
torn reads or concurrent reads and writes of unaligned values, use mutual
exclusion when getting these values (writes do, understandably, use them
already).

Signed-off-by: Erik Skultety <eskultet@redhat.com>