Convert ssrc_ctx_put() and ssrc_ctx_hold() to macros to aide in
debugging reference leaks. Having them as inline functions hides the
actual location of the refcount changes.
Obsolete ssrc_ctx_get() as returning values from macros is awkward and
it was only used in two locations.
Also fix a function invocation mishap: obj_get_o() should be used
instead of __obj_get() as the latter is an internal macro.
Change-Id: Icc0d63f04b3816632fd120c1c749cafabbbfa331