More debug of netflow_scan_and_export worker.

Output string of markers why scan export is terminated:

  s - hash table is empty (started but never entered the loop);
  T - trylock of hash based flow bucket is failed;
  E - all exported successfully (loop is exhausted);
  A - active flows ahead (because flow list is sorted by activity time).

Markers are recent to older from left to right in "Flows" line.