Add ability to immediately shutdown executor

Switch the task queue to a priority queue and allow
ShutdownThreadRequest (which originally were just the QUEUE_END_SENTINEL
object) to have a more important priority in the case that a user
has hit Ctrl+C.

This is also needed to fully fix: https://github.com/aws/aws-cli/issues/657

Previously, not all the integ tests were passing.  Now I can all unit/integ tests passing with this change.