Reviewed-by: Kishan
Changes:
- Separated out the External Network Usage task from the ExternalLBDeviceMgr because ExternalLbDeviceMgrImpl :: start() was getting multiple times during management server satrtup. The reason for this is that this is the baseclass for F5 and NetScalarElement.
- This caused us to schedule the ExternalNetworkUsageTask multiple times
- Also we have LBRulesMgr calling this ExternalLbDeviceMgrImpl by creating an instance of this class which is declared abstract
- Hence having a separate implementation to manage the network usage stats should solve this.