BUG-4084: Li:Save flows in operational based on barrier success

BUG-4084: Li:Save flows in operational based on barrier success

 - added ItemLifeCycleSource as the basic unit sourcing changes for
   DS/operational
 - this unit is provided by registered rpc services and if statistics
   are disabled then those units are listened to in order to keep device
   reflection in DS up to date
 - TODO: this covers flows only, groups and meters are on the line

 - from Jozef Gloncak: small fix of failing junit test
 - from Jozef Gloncak: change in test TimeCounterTest:
     Time marks are set in times 2 ms, 4 ms, 9 ms.
     Waited average time are 2/1 = 2 ms, 4/2 = 2 ms, 9/3 = 3 ms
     But this times are only theoretical if whole test is executed
     without latency and atomically. Therefore awaited average times
     can't be compared to exact values of awaited average time.
     (therefore == was replaced with >=)

Change-Id: Ie52ad4421745b09db68dec5c9b1be1b64d04d697
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>