consumerd: clean-up: stream attribute accessed without locking stream

consumer_metadata_cache_flushed makes use of the metadata stream's
ust_metadata_pushed attribute without locking while it is updated by
commit_one_metadata_packet() which holds the metadata stream lock.

This is marked as a clean-up since the attribute appears to always be
accessed while the metadata cache lock is held. However this is a
_channel_ attribute and the stream and channel lifetimes do not match,
making the locking assumptions conceptually dubious.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>