Changed buffer-per-socket to buffer-per-operation.

This is a difficult tradeoff to measure. I think large
datagrams won't be used very frequently. So it is better to
not lock a large buffer to each socket object and instead
allocate and deallocate for each operation receiving a
datagram larger than UDP_DATAGRAMSIZE.

test/testclnt.lua

@@ -669,7 +669,6 @@ local udp_methods = {
     "settimeout"
 }
 
-
 ------------------------------------------------------------------------
 test_methods(socket.udp(), udp_methods)
 do local sock = socket.tcp6()