tsnet: share a netcheck.Client among many tsnet.Servers?

If a process has a bazillion tsnet.Servers in it, they shouldn't all be doing their own netchecks to find a home DERP, etc.

We should probably fine a way to share it among all the tsnet instances.

(clearly magicsock, controlclient etc too, but those are harder, so this issue is specifically about netcheck which should be easy)

/cc @raggi @tailscale/client-core @lkosewsk @jasonodonnell