Change NotificationQueue assert -> CHECK

Change NotificationQueue assert -> CHECK

Summary: If the eventFd goes bad, NotificationQueue consumers can cause their event loop to spin indefinitely.  If libevent gets POLLERR on the fd, it calls handlerReady, which triggers a read.  The read will fail, but the callback is never uninstalled.  I'm not sure it would be easy to recover gracefully from this, so crash hard instead.

Reviewed By: yfeldblum, pgriess

Differential Revision: D2550270

fb-gh-sync-id: c9575fbda778b3625da6d9db6b0542b8263f230a