Cherry-picking VOL-3419 and releasing 2.5.6

VOL-3419: OpenOLT adapter at scale constantly takes more that 10 seconds to react to flows
The patch addresses the following
- Create OpenOltFlowMgr per PON port (instead of one instance for the whole OLT device earlier)
- Create a separate OpenOltGroupMgr - currently one instance for the whole OLT device
- Remove redundant global lock around getting ONU-ID in DeviceHandler module as there exists a
  separate per-pon-port lock in ResourceManager module which suffices the required synchronization
- Remove redundant locks in OpenOltFlowMgr module to serialize FlowDelete before FlowAdd
- Rename divideAndAddFlow to processAddFlow. "divideAndAddFlow" was used in 1.x voltha days and
  had a different meaning and the name seems to have been blindly ported to 2.x adapter
  and does not make sense anymore

Change-Id: I99827963cf242f1db0c27943c97bd05b749ae129
diff --git a/VERSION b/VERSION
index 0cadbc1..da6b0a8 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.5
+2.5.6