Since we panic when we see our root reorg out, even if we're not doing
anything yet, restoring the 100 block margin is the simplest fix.
Unfortunately this means adding a 100-block spacer in the tests, so things
don't get confused.
Fixes: #511
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>