Move the check for if a Maintenance script supports --batch-size away from addDefault...

Move the check for if a Maintenance script supports --batch-size away from addDefaultParams()
and to setBatchSize(), since setBatchSize is usually called in a subclass constructor after
parent:__construct() which is after addDefaultParams() is called.

Upside of this is lots of scripts supporting --batch-size now list it in their help output.