'Validates that supplied access token is registered and valid.
:param client_key: The client/consumer key.
:param token: The access token string.
:param request: An oauthlib.common.Request object.
:returns: True or False
Note that if the dummy access token is supplied it should validate in
the same or 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 AccessToken
try:
return AccessToken.exists(client_key, access_token)
except DoesNotExist:
return False
Rather than::
from your_datastore import AccessToken
if access_token == self.dummy_access_token:
return False
else:
return AccessToken.exists(client_key, access_token)
This method is used by
* ResourceEndpoint'

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

    raise NotImplementedError(u'Subclasses    must    implement    this    function.')
