Add LegacyPdoSessionHandler class

Addresses #12833.

The changes made to the PdoSessionHandler in 2.6 introduced a
backwards-compatability break for users upgrading from 2.5. This update
introduces a LegacyPdoSessionHandler class that uses the old service's
functionality.

Users who cannot make schema updates or do not want to lose sessions can
use LegacyPdoSessionHandler until 3.0.