If a conn.Close call raced conn.ReadFromUDPAddrPort before it could
"register" itself as an active read, the conn.ReadFromUDPAddrPort would
never return.
This commit replaces all the activeRead and breakActiveReads machinery
with a channel. These constructs were only depended upon by
SetReadDeadline, and SetReadDeadline was unused.
Updates #16707
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Jordan Whited
3d1e4f147a