Fix a bug where DB() tried to set the MySQL-specific 'sql_mode' on all drivers

Supersedes PR #2756
Issue #2763 - Fixes Session GC Probability Calculation

This should resolve issue #2763 where the cookie session garbage
collection was running every request.
Merge pull request #2764 from refringe/feature/cookie_session

Issue #2763 - Fixes Session GC Probability Calculation
DB call_function() bug : funny typo -or- smart autocomplete?
Merge pull request #2770 from kakysha/themostawesomebranchevercreatedforcodeigniter

DB call_function() bug : funny typo -or- smart autocomplete?
Fix #2775 (security lib docs)
Merge branch 'develop' of https://github.com/EllisLab/CodeIgniter into develop