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
[HttpFoundation] removed wrong HTTP header
Merge branch '2.3' into 2.5

* 2.3:
  [travis] reorder matrix + prevent "Killed" jobs
  [HttpFoundation] removed wrong HTTP header
Merge branch '2.5' into 2.6

* 2.5:
  [travis] reorder matrix + prevent "Killed" jobs
  [HttpFoundation] removed wrong HTTP header
Merge branch '2.6' into 2.7

* 2.6:
  [travis] reorder matrix + prevent "Killed" jobs
  [HttpFoundation] removed wrong HTTP header
Merge branch '2.7'

* 2.7:
  [travis] reorder matrix + prevent "Killed" jobs
  [FrameworkBundle] Optimize validator registration
  [FrameworkBundle] fix browserkit requirement
  [FrameworkBundle] fix expression-language requirement
  [TwigBridge] fix form requirement
  [HttpFoundation] removed wrong HTTP header
  [Filesystem] restore ability to create broken symlinks
  [FrameworkBundle] fix http-foundation requirement
  [CssSelector] added the license of the Python library we ported to PHP
  [SecurityBundle] avoid unneeded work