Add request filters for checking a request needs to be pinned

Move original request method check logic to a new module named `filters.py`, users can specify a list of filters named `settings.MULTIDB_REQUEST_FILTERS`, the default is `('multidb.filters.request_method_filter',)`.

I think it is necessary when the POST method does not always mean it should be pinned.
In 1.7 this is `import_string`, we should be forward-compatible here and do:

```python
try:
    from django.utils.module_loading import import_string
except ImportError:
    from django.utils.module_loading import import_by_path as import_string
```

(AFAICT the biggest difference is they throw different exceptions, and I'm not worried about that.)