Replace shaping instead of delete/create

[zealws]

We should be smarter about how we manage shaping. If someone calls shape twice the second call shouldn't unshape and re-shape, instead it should be smart and replace only the pieces that have changed.

This is pretty easy given the rich tc API we're using but the iptables API isn't as full featured so we'll have to be clever here.

Might depend on #173.

[chantra]

the good thing about replacing is that we would not need to change the mark so iptables is not affected here

[chantra]

I looked into this the other day and it was less straightforward that I expected. I should re-assess