Fix: src.ctf.lttng-live: decode metadata even on _STATUS_CLOSED

When receiving a _GET_ONE_METADATA_STATUS_CLOSED from the
`lttng_live_get_one_metadata_packet()` function it means that the
metadata stream was closed by the relay and we will no longer get any
new metadata. But, it also means that everything we received so far is
valid and can be decoded.

So rather than returning _LIVE_ITERATOR_STATUS_END the right thing to do
is to go on with the decoding of the metadata packets.

Also did the following cleanups:
  * Merge the `lttng_live_metadata::trace` and
  `lttng_live_trace::new_metadata_needed` into a new tri-state enum
  `lttng_live_metadata_stream_state` in the `lttng_live_trace` struct.
  Those two fields were tightly linked and it's simpler to have a single
  field.

  * Remove the "poking of the trace metadata" using the
  `new_metadata_needed` as it's no longer necessary since all references
  are now borrowed.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I3e18365d5cfeaa77935409bdfe8fdda52fa5b636
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2351