PdoSessionHandler gets its PDO connection via constructor parameter and there is a known PDO behavior that it connects to database at instantiation.

This cause a bad application behavior that every request opens a connection to database and authorize db user even if there is no need to create a session. (Think about a site which requires sessions rarely)

This is not a case when using Memcached or Mongodb because they have the ability to connect on demand.

I have a PR in progress, but first i'd like to get a confirmation to continue the work with unit tests and documentation too.