Datanode: _send_packet: fix a bug in flow-control logic
Datanode: _send_packet: fix a bug in flow-control logic

The flow control logic would wait for outstanding acks if the window grew
too large.  However, it would always invoke the v1 wait_ack routine, even
after v2 datanode support was added.  If encountered, this would have
resulted in an assertion failure error message and program exit.

cemeyer/hadoofus,cemeyer/hadoofus