Re-initialize the serial port object if reading becomes slow again

[smoser]

We had recently problems where read operations from the flarm became veeery slow. Restarting the client resolves the problem, but all queued messages are lost then. Unable to trace down the problem in the client, but taking two preventive measures:

• re-initialize the serial port object after n read operations
• re-initialize the serial port object if the read operations become slow again