lib: fix order of northbound operations

lib: fix order of northbound operations

When ordering operations, destroys must always come before other
operations, to correctly cover the change of a "case" in a "choice".

The problem can be reproduced with the following commands:
```
access-list test seq 1 permit 10.0.0.0/8
access-list test seq 1 permit host 10.0.0.1
access-list test seq 1 permit 10.0.0.0/8
```

Before this commit, the order of changes would be the following:
- `access-list test seq 1 permit 10.0.0.0/8`
  - `modify` for `ipv4-prefix`
- `access-list test seq 1 permit host 10.0.0.1`
  - `destroy` for `ipv4-prefix`
  - `modify` for `host`
- `access-list test seq 1 permit 10.0.0.0/8`
  - `modify` for `ipv4-prefix`
  - `destroy` for `host`

As `destroy` for `host` is called last, it rewrites the fields that were
filled by `modify` callback of `ipv4-prefix`. This commit fixes this
problem by always calling `destroy` callbacks first.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>