Fix: add more conditions and a lock to metadata cache flushed

Fix: add more conditions and a lock to metadata cache flushed

Some more conditions applies to return if the metadata cache is indeed
flushed. If no metadata stream is found or the endpoint status is set to
something other than ACTIVE, this means the channel is being (or will
be) destroyed thus the metadata cache can't be updated so is indeed
"flushed".

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>