'Validates that supplied request token is registered and valid.
:param client_key: The client/consumer key.
:param token: The request token string.
:param request: An oauthlib.common.Request object.
:returns: True or False
Note that if the dummy request_token is supplied it should validate in
the same nearly the same amount of time as a valid one.
Ensure latency inducing tasks are mimiced even for dummy clients.
For example, use::
from your_datastore import RequestToken
try:
return RequestToken.exists(client_key, access_token)
except DoesNotExist:
return False
Rather than::
from your_datastore import RequestToken
if access_token == self.dummy_access_token:
return False
else:
return RequestToken.exists(client_key, access_token)
This method is used by
* AccessTokenEndpoint'

def validate_request_token(self, client_key, token, request):

    raise self._subclass_must_implement(u'validate_request_token')
