Bug 1764 - moved Session related interfaces to openflowplugin-api

- session related interfaces moved from implementation to api
- Extension related methods erradicated from SessionManager and
  moved to extension-api's ExtensionSessionManager
- openflowplugin-api SessionManager and openflowplugin-extension-api ExtensionSessionManager
  joined for openflowplugin neeeds in ConjuctSessionManager to provide functionality
  of both apis

Change-Id: I947a59714900cb7e6d2fb7376c4d743e1764ac7b
Signed-off-by: Martin Bobak <mbobak@cisco.com>