FlowEntry must not be modified outside store.

- Remove set method from FlowEntry
- Storing last seen timestamp for FlowEntry eviction locally on FlowManager.
  FlowEntry eviction based on packet counter will take longer time to timeout
  after master Node change.

Change-Id: I7134d698dd5b9bf7cca379c5ba7c4fbcc2e3d5f3
7 files changed