Fix: protect the channel's metadata stream using the metadata cache lock

The consumer_thread_data_poll and consumer_thread_metadata_poll
both access the channel's metadata stream.

During a session destruction, consumer_thread_metadata_poll will
destroy all metadata streams. However, the consumer_thread_data_poll
may still invoke a consumer_metadata_cache_write() triggered
by a "ready" subbuffer. Hence, the metadata stream must be protected
from this action by the metadata cache lock.

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