Add newlink/delink processing.

- Can up/down a link.
- Can set MAC on a link, if it's a phy.
- Can set MTU on a link.
- Can delete link (including phy).

Because link state and mtu changes tend to go around in circles (from
netlink -> vpp; and then with lcp-sync on, as well from vpp -> netlink)
when we consume a batch of netlink messages, we'll temporarily turn off
lcp-sync if it's enabled.

TODO (in the next commit), the whole nine yards of creating interfaces
in VPP based on NEWLINK vlans that come in. Conceptualy not too
difficult: if NEWLINK doesn't have a LIP associated with it, but it's a
VLAN, and the parent of the VLAN is a link which _does_ have a LIP, then
we can create the subint in VPP in the correct way.

README.md

@@ -44,10 +44,10 @@ column shows changes in LInux that are copied into VPP.
 
 | Function       | VPP -> Linux  | Linux -> VPP |
 | -------------- | ------------- | -------------|
-| Up/Down Link   | ✅            | 🟠            |
-| Change MTU     | ✅            | 🟠            |
-| Change MAC     | ❌ 1)         | 🟠            |
-| Add/Del IP4/IP6 Address  | ✅  | 🟠            | 
+| Up/Down Link   | ✅            | ✅            |
+| Change MTU     | ✅            | ✅            |
+| Change MAC     | ❌ 1)         | ✅            |
+| Add/Del IP4/IP6 Address  | ✅  | ✅            | 
 | MPLS           | ❌            | ❌            |
 | Route          | ❌ 2)         | 🟠            |
 | Add/Del Tunnel | ❌            | ❌            |