perf(select): execute render after $digest cycle

This is an optimization to defer execution of the render function in the
select directive after the $digest cycle completes inside the
$watchCollection expressions.  This does a check to see if the render
function is already registered in the $$postDigestQueue before it passes
it into $$postDigest, guaranteeing that the DOM manipulation happens
only in one execution after the model settles.

Closes #8825