Cache Maintain zones to improve lookup speed

I probably shouldn't have wasted time on this, but I DID IT ANYWAY. And regardless it was pretty easy.


master (88feacb):
```none
   ncalls  tottime  percall  cumtime  percall
    11118    0.200    0.000  954.430    0.086
```

dhcp_migrate-speed (43e1471):
```none
   ncalls  tottime  percall  cumtime  percall
    11118    0.071    0.000   18.305    0.002
```