The unit test showed a bug with the stopping of the watchdog thread in case of error.
The watchdog thread is now handled at the same level as all thread, at the session level.

The disk write task can already be destroy by the time the push of NULL is attempted.

Fixed mother forker passing its own pid instead of the session's as the jid for the vdqm when reported tape mounted.

Added support for cancellation of write tasks. This is used to flush tasks when tape read session is lost after a failed positioning.

The logs of the read session also contain statistics also in case of failed session.

Only one applies at a time in read or write context, but the naming will make the code more undrestandable.

Fixed in the trunk.

Added a check for fresh block cleanliness in getFreeBlock (for both DataFifo in migrations and RecallMemoryManager).