Fixing issue phalcon/incubator#22 and other improvement of Phalcon\Session\Adapter\Database

SQL-queries were changed to prevent possible syntax errors and injections.
Sessions garbage collector callback was implemented.
The source code was formatted with php-cs-fixer.
Fixing issue phalcon/incubator#18
Merge pull request #35 from endeveit/master

Fixing issue phalcon/incubator#22 and other improvements