block: support for block number >32 bit (Fix #968)
block: extend generic driver API (Ref #750)

* allow to handle a maximum of packets in parallel
  that fits free slots in the ack queue
* stop processing packets, when the driver can't handle
  more requests in parallel, and resume packet handling,
  when the driver is ready again
packet stream: clean range allocator in destructor
block: extend block session test framework
part_blk: re-design to work event driven

Fix #750