rdbms: add replica server counting methods to ILoadBalancer

This is slightly more robust and makes the intent much clearer
than random calling code checking getServerCount() all over the
place. In addition, this yields better separation of concern.

Also, cleanup the LoadBalancer constructer a bit and make the
validation a bit stricter.

Make some server index comparisons strict while at it.

Change-Id: Icc1a35bd65c6862ff81faa3ab9b2aa7cafe29443