Interestingly, this patch and the last take my total test time on my
build machine down by about 10%. From 403.93s (0:06:43) to 366.64s (0:06:06)
though there were some unrelated errors.
Changelog-Changed: connectd: I/O optimizations to significantly speed up larger nodes.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>