Make smart shutdown work in combination with Hot Standby/Streaming Replication.

At present, killing the startup process does not release any locks it holds,
so we must wait to stop the startup and walreceiver processes until all
read-only backends have exited.  Without this patch, the startup and
walreceiver processes never exit, so the server gets permanently stuck in
a half-shutdown state.

Fujii Masao, with review, docs, and comment adjustments by me.