* 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
To support components, which implement the block session's server side
rpc object, and which doesn't write data to their device backend immediately,
an additional synchronization call is needed. Thereby, clients like for
instance a file system can tell these components, when a synchronization is
required.
Ref #113
