feature: local firmware update

[JanZachmann]

• introduced local firmware update via web_service
  • new feature FirmwareUpdate providing load and run update api
  • extended update_validation to handle the "local scenario" where no iothub connection might be present
• fixed bug in main select! loop:
  • changed former solution with outer and inner select! which tried to have first a set of prioritized branches (1st select!) before having the 2nd select! with random access branch selection. The 2nd select! was canceled as soon as there have been other events in 1st select!. As a consequence running commands were canceled as well what caused unexpected behavior.
  • no there is only one prioritized select! statement but therefore there is one dedicated stream for CommandRequests
  • CommandRequests are used for all internal and external requests now. Thus we can use a singe stream for all of them.
• fixed bug when IotHubClient::shutdown() took too long which resulted in a watchdog timeout