Until now, block drivers had to deal with a pointer to the client
session component, e.g.: to acknowledge block packets already processed.
When a session was closed, the driver object wasn't informed explicitly,
which leads to defensive programming, or lastly to a race-condition in
test-blk-srv. To prevent from this class of errors, the pointer is now
private to the generic block driver base class, and not accessible to
the concrete driver implementation. Moreover, the driver gets explicitly
informed when a session got invalidated.
Ref #113
eeb2d95b1f