Major bug fix:

The server needs to zero out the key in the polled request immediately
after detecting it. It cannot defer this to the time when the corresponding
response is sent. The reason for this is pretty simple.

Consider the following state of the pipeline. R(ci) denotes a request from
client i. The request region for client 0 is initially empty

Pipeline:			A(c1)	---->	B(c1).
C0 Req Region:		X				X

Now, a request from C0 comes along. This causes B(c1) to get flushed from the
pipeline.

Pipeline:			C(c0)	---->	A(c1).
C0 Req Region:		C(c0)			X

Another request from C0 comes along. This causes A(c1) to get flushed.

Pipeline:			D(c0)	---->	C(c0).
C0 Req Region:		C(c0)			D(c0)
					||	<- Server polls here
At this point, the server sees C(c0) again if its key has not been zeroed out.

So, we need to zero out keys. Currently, the last 8 bytes of the key are stored
along with the pipeline item.