Consumer: Implement lttng_consumer_clear_channel

This function is responsible for performing all actions needed to
clear a given channel.

It only supports clear operation on unmonitored channel
(snapshot mode) for now.

To do so, flush and clear all the channel streams.

We use an active flush (consumer_flush_buffer(..., 1)) since we consider
the producer active at all time. No reason so far to check for the
quiescent state of the channel. This might need to be revisited.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>