Partial conversion of registrar to not keep a coro alive for the whole span of the registration.
Remaining conversion of registrar to not use coroutines, plus coro pool size tuning. (currently still buggy)
Fixes bug in disconnect_watcher_t that was triggered if it was the last thing keeping a connection alive.
Improvements in non-coroutine registrar.
Merge branch 'next' into daniel_2211_nocoro_registrar