This commit also introduces a sync.Mutex for guarding mutatable fields
on serverEndpoint, now that it is no longer guarded by the sync.Mutex
in Server.
These changes reduce lock contention and by effect increase aggregate
throughput under high flow count load. A benchmark on Linux with AWS
c8gn instances showed a ~30% increase in aggregate throughput (37Gb/s
vs 28Gb/s) for 12 tailscaled flows.
Updates tailscale/corp#35264
Signed-off-by: Jordan Whited <jordan@tailscale.com>
a663639bea