Support multiple consumer queues.

- Add support for importing posted buffers when spawing a new
  consumer queue.
- Correctly handle adding signaled buffers to epoll with edge
  triggered mode set.
- Add test for multi-consumer behavior.

Bug: 36401174
Test: buffer_hub_queue-test passes.
Change-Id: Id09f01502a1b18bf80a0ae465c2941b548cde2e4