feature/conn25: expire idle flows from FlowTable

Track lastSeen on each cached flow and add a sweeper goroutine
that periodically removes flows idle past the idle timeout.

Introduce tunables for idle timeout, maximum flows removed per sweep (to
limit mutex hold time), and the sweeper interval.

Also cap the previously-unlimited tables: 10k client flows, 100k
connector flows.

Updates tailscale/corp#38630

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>

feature/conn25/conn25.go

@@ -134,6 +134,7 @@ func (e *extension) Init(host ipnext.Host) error {
 	ctx, cancel := context.WithCancelCause(context.Background())
 	e.ctxCancel = cancel
 	go e.sendLoop(ctx)
+	dph.StartFlowExpirySweepers(ctx)
 	return nil
 }