Fix: consumerd metadata channel/cache/timer races

There were various races with the way metadata was handled for UST. Fix
this by:

- Ensuring that metadata cache is destroyed only when the metadata
  channel is destroyed (it is already protected by refcounting).
- Ensuring that close_metadata() command (issued by sessiond) just
  closes the stream wakeup_fd, and let the metadata management thread
  (which has ownership of the metadata stream) perform the teardown.
- Ensure that switch timer, for metadata, is always stopped before any
  of the metadata stream or channel teardown is done.

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