Make ChronologyProtector actually use cpPosTime cookies

Make ChronologyProtector actually use cpPosTime cookies

Setup.php was only injected the value from $_GET. LBFactory used
to check both before 0e5cd18b74d83c50, with the cookie check left
in LBFactoryMW. After d175b391ae2a7, LBFactoryMW class switched to
the default LBFactory method for making a ChronologyProtector
instance, so the cookie check was lost there too.

This means that an HTTP GET request due to a redirect from a POST
request will still make sure the DB position store is up-to-date.
This only matters is that store uses replication (e.g. cross DC).

Also refactor LBFactory handling of the cpPosTime URL parameter.

Change-Id: Ie5744577609f4c193cacd71334f92b30f02e3846