We used to create the entire reply, the if it was too big, split in
half and retry.
Now that the main network is larger, this always happens with a full
request, which is inefficient.
Instead, produce a reply assuming no compression, then compress as a
bonus. This is simpler and more efficient, at cost of sending more
packets.
I also renamed an internal dev var to make it clearer.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>