Fix: don't steal key when adding a metadata stream

This was causing a stream corruption of the node key if the stream->key
of the metadata was matching a stream wait_fd making the stream not
findable and asserting when getting out of the metadata poll wait.

Now we lookup the stream before adding it to make sure it's unique and
don't try to steal the key anymore since wait_fd is unique to the
consumer.

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