optimize slow oauth2_authorizations query that runs frequently

e.g.:
    SELECT * FROM `oauth2_authorizations`
    WHERE `oauth2_resource_owner_id` = 123456789
    AND `oauth2_resource_owner_type` = 'User'
    AND `client_id` = 2 LIMIT 1

uses the wrong index and has to scan half the table (~1 second to run)
Merge pull request #1 from lumoslabs/oa2a_add_index

optimize slow oauth2_authorizations query that runs frequently