Improve flow collection

Improve flow collection

- Delete all known flows synchronously
- Add boolean isTransactionsEnabled to TxFacade to determine if it was
initialized and initial submit was called
- Improve deletion of flows during on the fly statistics gathering (now,
  each gathering have 3 states, INITIALIZATION, WORKING and TERMINATION
  and behaves according to that)
- Fix possibility of statistics collection freeze when error occurs
  during OF Java to OF Plugin model translation while gathering
  statistics on the fly

Resolves: bug 7501

Change-Id: I74843d4536215c192ac43964634502430da1aa9a
Signed-off-by: Tomas Slusny <tomas.slusny@pantheon.tech>