EventHubTransmitter dispose issues

[glucaci]

Random exeption when disposing EventHubTransmitter:
System.ObjectDisposedException : The CancellationTokenSource has been disposed.
on line _disposeToken?.Dispose();

Also observed random exception in the same dispose method on:
_transmission?.Dispose(); with exception A task may only be disposed if it is in a completion state (RanToCompletion, Faulted or Canceled)

[glucaci]

Dispose reworked through #70. Remain open till further testing will be done.