Shutdown must complete even if Allocator eats interrupt signal

If the interrupt happen inside the Allocator.allocate(Slot) method, and
then the allocator either clears the interruption status, or throws an
InterruptedException which in turn will be understood as poison, then the
allocation thread can miss the shutdown signal, and never begin the
shutdown sequence.
Merge pull request #67 from chrisvest/bug/shutdown-interrupt

Shutdown must complete even if Allocator eats interrupt signal