Long contrail-api startup time because of populating ifmap

[Ankudinov Alexander]

On database with about 10000 VMs contrail-api restart takes 18 minutes.

Most of this time is wasted in populating ifmap because of two things:
* redundant conversion from str to unicode and back
* bug in contrail-api leading to redundant conversion of large dicts to str

About dead code:
In line 716 of `vnc_ifmap_client.py` there is an error:

```
old_metalist.append(self_metas[meta_name])
```

instead of:

```
if id2 in self_metas[meta_name]:
    old_metalist.append(self_metas[meta_name][id2])
```

So `old_metalist` contains dicts instead of `Metadata`.

As a consequence, if `request != old_request` later in the code is always true.

[OpenContrail Admin (ci-admin-f)]

Review in progress for https://review.opencontrail.org/48075
Submitter: Ankudinov Alexander (<email address hidden>)