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

def validate_client_key(self, client_key, request):

    raise self._subclass_must_implement(u'validate_client_key')
