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>

shell.nix

@@ -16,4 +16,4 @@
 ) {
   src =  ./.;
 }).shellNix
-# nix-direnv cache busting line: sha256-rV3C2Vi48FCifGt58OdEO4+Av0HRIs8sUJVvp/gEBLw=
+# nix-direnv cache busting line: sha256-pZCy1KHUe7f7cjm816OwA+bjGrSRnSTxkvCmB4cmWqw=