Ignore memb_join messages during flush operations

a memb_join operation that occurs during flushing can result in an
entry into the GATHER state from the RECOVERY state. This results in the
regular sort queue being used instead of the recovery sort queue, resulting
in segfault.

Signed-off-by: Steven Dake <sdake@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>