Restore proxyDHCP log throttling

The per-MAC 90 second log throttle in proxydhcp has been inert: the
`skiplogging = True` reset sat in relay_proxydhcp, where it is a dead
local, while the loop in proxydhcp only ever assigns False.  Once the
first packet is handled the flag stays False for the life of the
process, so every retransmitted boot request logs again even though
ignoredisco is updated to suppress it.

Reset the flag at the top of each loop iteration instead, next to the
timestamp check it belongs to, and drop the dead assignment.

confluent_server/confluent/discovery/protocols/pxe.py

@@ -279,7 +279,6 @@ def relay_proxydhcp(sock, pktq):
     elif disco.get('uuid', None) in uuidmap:
         node = uuidmap[disco['uuid']]
     myipn = myipbypeer.get(data[28:28+hwlen], None)
-    skiplogging = True
     pktq.put_nowait((disco, peer, myipn, idx, recv, node, opts, data))
 
 
@@ -296,6 +295,7 @@ async def proxydhcp(handler, nodeguess):
         try:
             disco, client, myipn, idx, recv, node, opts, data = await pktq.get()
             netaddr = disco['hwaddr']
+            skiplogging = True
             if time.time() > ignoredisco.get(netaddr, 0) + 90:
                 skiplogging = False
                 ignoredisco[netaddr] = time.time()