The new request tag allows a block-session client to uniquely correlate
acknowledgements with outstanding requests. Until now, this was possible
for read and write operations by taking the value of the request's
packet-stream offset. However, SYNC and TRIM requests do not carry any
packet-stream payload and thereby lack meaningful offset values. By
introducing the notion of a 'tag', we can support multiple outstanding
requests of any type and don't need to overload the meaning of the
'offset' value.
Issue #3274
e058adc4fe