Add deprecation warning to LegacyPdoSessionHandler
Add deprecation warning to LegacyPdoSessionHandler

LegacyPdoSessionHandler has been deprecated in favor of
PdoSessionHandler. Trigger an E_USER_DEPRECATED error when the
deprecated class is used.

ygerasimov/HttpFoundation,oswaldderiemaecker/HttpFoundation,symfony/HttpFoundation,symfony/http-foundation,mneuhaus/HttpFoundation,nichdu/HttpFoundation