rdbms: make sure cpPosIndex cookie is applied to LBFactory in time

rdbms: make sure cpPosIndex cookie is applied to LBFactory in time

Once getMain() was called in setSchemaAliases(), the ChronologyProtector
was initialized and the setRequestInfo() call in Setup.php had no effect.
Only the request values read in LBFactory::__construct() were used, which
reflect $_GET but not cookie values.

Use the $wgDBtype variable to avoid this and add an exception when that
sort of thing happens.

Further defer instantiation of ChronologyProtector so that methods like
ILBFactory::getMainLB() do not trigger construction.

Bug: T192611
Change-Id: I735d3ade5cd12a5d609f4dae19ac88fec4b18b51