if we ever wanted to time out idle threads.)

let overlays define if they must appear exactly once in a chain, and if they must be global/per-db only, and holler in case

here since the script has to respawn the servers anyway.

the event set must be removed using slapd_remove() and the whole sequence
must be protected with the slap_daemon.sd_mutex to close them properly.
Otherwise it will wind up closing valid, freshly opened sockets, or closing
dead sockets twice, etc. It's unnecessary to do any close management here
since connection.c already does the correct teardown sequence.