ref(value)!: atomic decref result check

[jpnurmi]

Note

SC/BC break (low impact)

Fixes TOCTOU races in sentry__transaction_context_free, sentry__transaction_decref and sentry__span_decref by adding return value for sentry_value_decref indicating whether the value still has references, has been freed, or is primitive that needs no tracking:

if (!sentry_value_decref(value)) {
  free(related_resources);
}

While taking the BC/SC break hit, make sentry_value_incref return the incref'd value for convenience, to avoid reducing such boilerplate:

// before
sentry_value_incref(value);
do_something(value);

// after
do_something(sentry_value_incref(value));