[DB MIGRATION REQ'D] Add database $_SESSION storage
Creates SessionDAO Interface and MySQL implementation
Creates sessions table
Adds SessionCache::init() to start sessions and set handlers
Modifies controllers to make use of this
Moved date_default_timezone_set($config->getValue('timezone')); before SessionCache::init call.
   The DateTime construct error was happening because in ThinkUpController, SessionCache::init was being called in ``__construct``, before ``initializeApp``` is called in ``go``. The default timezone is set in ``initializeApp``.

Fixes for failing tests from Session Changes
  Handle new tables in installer test
  Fix wrong table count in mysql install dao test.

Adds new config value use_db_sessions which defaults to true
  Add new documentation

Closes #1818, closes #962