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)
Cleanup DB_forge _process_indexes
Cleanup DB_forge _process_indexes
Merge pull request #2777 from mjnaderi/develop

Cleanup DB_forge _process_indexes
Use table name as a prefix for index names
Merge pull request #2779 from mjnaderi/develop

Use table name as a prefix for index names
[ci skip] Add notes to get_compiled_insert(), get_compiled_update() docs
[ci skip] Fix a changelog message
EXISTS / NOT EXISTS clause support for  DB_query_builder.
EXISTS / NOT EXISTS clause support for  DB_driver
EXISTS / NOT EXISTS clause support for  DB_driver
changed EXISTS / NOT EXISTS pattern
@narfbg "It has parenthesis, so use the IN / NOT IN pattern as a base."
added changelog about EXISTS / NOT EXISTS
@narfbg "And you should also add a changelog entry for this addition. :)"
so we need \sNOT EXISTS
Merge pull request #2781 from tufanbarisyildirim/develop

EXISTS / NOT EXISTS clause support for  DB_driver