Previously, the echo test followed a flow like the following:

-STEP-	-HOST PC-                  -DEVICE-
0	send _sync
1				   echo back _sync
2				   send echo_count
3	echo back echo_count
4	send first echo packet
5				   echo back echo packet
	(repeat echo steps)

However, as noted by issue #6659, this test would somtimes fail between
steps 4 and 5. To ensure each KV pair makes to the correct destination,
we usually write the KV back. Step 4 does not wait for this to happen
and starts sending echo packets. So the device is acting as the "echo
server".

This change makes the host PC the "echo server". The idea being that the
device will be slower and the host pc should always be able to keep up
with it, not the other way around.