FRM should use async function on southbound event

- On reacting to southbound events (switch up/down, ports added/removed, ...)
  FRM is supposed to use asynchronous calls for flows installation/removal.
  This is needed because these events may affect a large number of flows,
  for which we do not want to slow down the processing via sync calls.
  Also, logic has been we never do process synchronously a southbound event.
- In case of port up, FRM was still using a synchronous call.
  This patch is for taking care of that.

Change-Id: I4b6dae0df75ad778ea2f1d555a258a48ea66c57b
Signed-off-by: Alessandro Boch <aboch@cisco.com>