datatypes: Introduce some admin-related close callback handling helpers

datatypes: Introduce some admin-related close callback handling helpers

Well, there were three different spots where closeCallback->freeCallback was
called, not looking the same --> potential for bugs - and there indeed is a bug
with refcounting of the @conn object. So this patch partially follows the path
set by commit 24dbb69f by introducing some close callback helpers both to
replace all the spots where we call clean the close callback data with a
dedicated function and to be able to fix the refcounting bug causing a memleak.

Signed-off-by: Erik Skultety <eskultet@redhat.com>