When a session has data pipelined we perform a dirty dance to move it at
the beginning of the workspace. The rollbacks used to occur between
HTC_RxPipeline() and HTC_RxInit() calls until it was centralized in the
latter.

With a dedicated WS_ReqPipeline() operation we can capture the semantics
of initializing an existing connection for its next task with or without
data fetched from the previous task.

While conceptually there is still a use-after-free since the pipelined
data may belong to the same workspace, it is fine if that happens within
the bounds of an atomic workspace operation.