SAR-239 Added long running command device client utility

This MR adds a utility class LongRunningDeviceInterface, for clients of devices that implement long running commands to simplify the submission of a command to execute.

Currently clients need to store the command ID of the long running command they executed. They also need to subscribe to attributes to keep track of the command states to determine whether the command completed.

In LongRunningDeviceInterface:

• Client supply a list of Tango devices to execute a command on.
• Client supply the command name and argument.
• Client supply a callback that will be executed once the command have completed on all those devices.
• The class will execute the command on each device.
• The class will then subscribe to the correct attributes and keep track of the command IDs.
• The class will fire the callback when the command completed on the devices.

JIRA: SAR-239