Refactor session callback logic into Session_Handler abstraction

[danielbachhuber]

Notably, this PR does two things:

• Removes the lazy session loading logic in _pantheon_session_write(). It was copied from Drupal in 86271db#diff-fd59fcd185e70b3120d36cdb1142dd63R80-R100 but the session starting was removed in 3045dcf#diff-fd59fcd185e70b3120d36cdb1142dd63L107. Setting session_id() will always error because the session needs to have been started by the time _pantheon_session_write() is called.
• Avoids registering a PHP sessions shutdown callback, because we already have our own hooked into WP's shutdown hook.

See #118