When is it safe to leave sc_writewait uninitialized? The field
seems to have emerged in the middle of RE24, and I don't see any
flags which say it should/should not be used.
Since not setting it crashed back-relay in ITS#8428, does that
mean everything which creates a slap_callback should set it?
Seems unfortunate for third-party modules...
Also, I think this should be documented.