Fix: consumerd channel destroy race

Fix: consumerd channel destroy race

The LTTNG_CONSUMER_DESTROY_CHANNEL command is used to handle errors. It
should not call destroy_channel(), because at that point the channel
management thread has ownership of the channel. Send a notification to
the channel management thread asking it to destroy the channel instead.

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