device: make RoutineReadFromTUN keep encryption queue alive

device: make RoutineReadFromTUN keep encryption queue alive

RoutineReadFromTUN can trigger a call to SendStagedPackets.
SendStagedPackets attempts to protect against sending
on the encryption queue by checking peer.isRunning and device.isClosed.
However, those are subject to TOCTOU bugs.

If that happens, we get this:

goroutine 1254 [running]:
golang.zx2c4.com/wireguard/device.(*Peer).SendStagedPackets(0xc000798300)
        .../wireguard-go/device/send.go:321 +0x125
golang.zx2c4.com/wireguard/device.(*Device).RoutineReadFromTUN(0xc000014780)
        .../wireguard-go/device/send.go:271 +0x21c
created by golang.zx2c4.com/wireguard/device.NewDevice
        .../wireguard-go/device/device.go:315 +0x298

Fix this with a simple, big hammer: Keep the encryption queue
alive as long as it might be written to.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>