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.
(My examples aren't edited for pep8/79-char-lines but should be in the final patch.)