Performance improvements for update-template fn

In order to improve performance of the pedestal push renderer
template helpers, i refactored the update-template function in
namespace io.pedestal.app.render.push.templates.

Implemented changes:

 - The function now walks only over fields which are actualy
   updated, instead of walking over all the fields in the
   stored dynamic template change map

 - Because of the first point, is no longer necessary to run
   contains? checks in each side-effect evaulation, which
   further improves performance

 - Because there are only two possible actions for :attr template
   keys, which is either to remove attr if it's set to nil or to
   set attr if it's set to every other value, i changed the control
   structure from using cond to simple if macro