audiounit: Avoid AudioCore deadlock in stream init (Bug 1317234)

* audiounit: Synchronize stream init/start/stop/destroy

* audiounit: Refactor switchin to a method

* audiounit: Use dispatch_queue for switching

* audiounit: Refactor input/output configuration

* audiounit: After changing buffer size wait to take effect

* audiounit: Set output buffer size using global latency

* audiounit: Set input buffer size using global latency

* audiounit: update review comments

* audiounit: Capitalize enum constant