Wait until server is closed and all connections are dropped.
- If the server is not closed, wait.
- If it is closed, but there are still active connections, wait.
Anyone waiting here will be unblocked once both conditions
(server is closed and all connections have been dropped)
have become true, in either order.
Historical note: In 3.11 and before, this was broken, returning
immediately if the server was already closed, even if there
were still active connections. An attempted fix in 3.12.0 was
still broken, returning immediately if the server was still
open and there were no active connections. Hopefully in 3.12.1
we have it right.
3.12: wait_closed() now waits until the server is closed and all active connections have finished. Previously, it returned immediately if the server was already closed, even if connections were still active.