Improve the performance of tokens deletion for user

Provide new delete the tokens api 'delete_tokens' to support
delete all the tokens for user in one session in the sql backend. For
the kvs and memcache, I also provide the corresponding implementation.

Fix bug 1178063

Change-Id: I986a583e5900ea04e26cbdb7c49638a33818bca7
(cherry picked from commit d6cfe4f2e2c7c1b3eb23bf9f864779940a645555)
Merge "Improve the performance of tokens deletion for user" into stable/grizzly