res_stasis: Reduce RAII_VAR usage.

In addition to being a micro-optimization (RAII_VAR has overhead), this
change improves output of REF_DEBUG.  Unfortunately when RAII_VAR calls
ao2_cleanup it does so from a generated _dtor_varname function.  For
example this caused _dtor_app to release a reference instead of
__stasis_app_unregister.

Change-Id: I4ce67120583a446babf9adeec678b71d37fcd9e5