a few bugfix: 1. update mongodbe last_activity in sess_update; 2. keep up with codeigniter 2.1.3 changes on sess_destroy; 3. do not affect mysql session id generation, substr when needed to generate mongodb objectID
do not use object id as session id, mongodb index can achieve similar lookup speed with a simple index on session_id field, with this change we completely avoid truncating md5 session id to 24 hex string, and we reduce the query needed in sess_update from 3 (get, delete, insert) to 1 (update)
fix silly mistake on where query
one fix oversight
fix issue on insert query side-effect
Merge pull request #2 from bitinn/master

Bugfixes, thanks to @bitinn.