net/tstun: buffer outbound channel

The handoff between tstun.Wrap's Read and poll methods
is one of the per-packet hotspots. It shows up in pprof.

Making outbound buffered increases throughput.

It is hard to measure exactly how much, because the numbers
are highly variable, but I'd estimate it at about 1%,
using the best observed max throughput across three runs.

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

net/tstun/wrap.go

@@ -153,7 +153,7 @@ func Wrap(logf logger.Logf, tdev tun.Device) *Wrapper {
 		// a goroutine should not block when setting it, even with no listeners.
 		bufferConsumed: make(chan struct{}, 1),
 		closed:         make(chan struct{}),
-		// outbound is buffered as an optimization.
+		// outbound can be unbuffered; the buffer is an optimization.
 		outbound:     make(chan tunReadResult, 1),
 		eventsUpDown: make(chan tun.Event),
 		eventsOther:  make(chan tun.Event),