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>

tsnet/depaware.txt

@@ -9,6 +9,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
  LDW    github.com/coder/websocket/internal/errd                     from github.com/coder/websocket
  LDW    github.com/coder/websocket/internal/util                     from github.com/coder/websocket
  LDW    github.com/coder/websocket/internal/xsync                    from github.com/coder/websocket
+        github.com/creachadair/msync/trigger                         from tailscale.com/logtail
    W 💣 github.com/dblohm7/wingoes                                   from tailscale.com/net/tshttpproxy+
    W 💣 github.com/dblohm7/wingoes/com                               from tailscale.com/util/osdiag+
    W 💣 github.com/dblohm7/wingoes/com/automation                    from tailscale.com/util/osdiag/internal/wsc