logtail: avoid racing eventbus subscriptions with shutdown (#17695)

In #17639 we moved the subscription into NewLogger to ensure we would not race
subscribing with shutdown of the eventbus client. Doing so fixed that problem,
but exposed another: As we were only servicing events occasionally when waiting
for the network to come up, we could leave the eventbus to stall in cases where
a number of network deltas arrived later and weren't processed.

To address that, let's separate the concerns: As before, we'll Subscribe early
to avoid conflicts with shutdown; but instead of using the subscriber directly
to determine readiness, we'll keep track of the last-known network state in a
selectable condition that the subscriber updates for us.  When we want to wait,
we'll wait on that condition (or until our context ends), ensuring all the
events get processed in a timely manner.

Updates #17638
Updates #15160

Change-Id: I28339a372be4ab24be46e2834a218874c33a0d2d
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

go.mod

@@ -21,6 +21,7 @@ require (
 	github.com/coder/websocket v1.8.12
 	github.com/coreos/go-iptables v0.7.1-0.20240112124308-65c67c9f46e6
 	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf
+	github.com/creachadair/msync v0.7.1
 	github.com/creachadair/taskgroup v0.13.2
 	github.com/creack/pty v1.1.23
 	github.com/dblohm7/wingoes v0.0.0-20240119213807-a09d6be7affa
@@ -114,7 +115,7 @@ require (
 	golang.zx2c4.com/wireguard/windows v0.5.3
 	gopkg.in/square/go-jose.v2 v2.6.0
 	gvisor.dev/gvisor v0.0.0-20250205023644-9414b50a5633
-	honnef.co/go/tools v0.5.1
+	honnef.co/go/tools v0.6.1
 	k8s.io/api v0.32.0
 	k8s.io/apimachinery v0.32.0
 	k8s.io/apiserver v0.32.0