Use slave for selects in CategoryMembershipJob

Use slave for selects in CategoryMembershipJob

This patch switches to using a slave but imediatly
waits for the slave to catch up with master
(so as not to miss things).

This may result in more delay between an edit and
category changes being inserted.

It may be possible to instead wait for the timestamp
that is passed in $this->params['revTimestamp']
which could result in slightly less delay.

I can't see any uses of waitForReplication in quite
this way but see no imediate reason this would not work.

Bug: T125147
Change-Id: Ia0aa722c97f41a3959bcd3cb4210b39db0c3bc45