Add LegacyPdoSessionHandler class

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.
Merge branch '2.6' into 2.7

* 2.6:
  Add LegacyPdoSessionHandler class
  fix dependency on ExtensionInterface over implementation
  [FrameworkBundle] forward error reporting level to insulated Client
  [VarDumper] fix and test PdoCaster
  [Config] fix error handler restoration in test