MINOR: proxy: rename default refcount to avoid confusion

MINOR: proxy: rename default refcount to avoid confusion

Rename proxy conf <refcount> to <def_ref>. This field only serves for
defaults proxy instances. The objective is to avoid confusion with the
newly introduced <refcount> field used for dynamic backends.

As an optimization, it could be possible to remove <def_ref> and only
use <refcount> also for defaults proxies usage. However for now the
simplest solution is implemented.

This patch does not bring any functional change.