Fix race between next_in_queue and balance_pool

  mv: rename /poudriere/data/build/91amd64-pkgng/ref/poudriere/pool/unbalanced/p5-Role-Tiny-1.003.002 to /poudriere/data/build/91amd64-pkgng/ref/poudriere/building/p5-Role-Tiny-1.003.002

Reported by:	mat
Fix race between queue_empty and balance_pool.

This may have been resulting in stuck queue crashes.

Reported by:	many