Add channel wakeup fd to monitor close

Add channel wakeup fd, so consumer can keep its handle on the stream
wakeup_fd (for periodic timer flush), and yet still discover that an
application has closed a channel or exited.

Requires to be updated in locked-step with lttng-ust
"Add channel wakeup fd to monitor close"

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>