Retry failed rebind attempts

When we receive a migration, if rebinding to the new listen address and
port fails for a reason which might be fixable, rather than killing the
server we retry once a second.  Also in this patch: non-overlapping log
messages and a fix for the client going away halfway through a sendfile
loop.

tests/acceptance/fakes/dest/close_after_entrust.rb

@@ -24,5 +24,6 @@ client.close
 client2 = server.accept
 client2.receive_mirror
 
+
 exit(0)