state/nodestore: in memory representation of nodes

Initial work on a nodestore which stores all of the nodes
and their relations in memory with relationship for peers
precalculated.

It is a copy-on-write structure, replacing the "snapshot"
when a change to the structure occurs. It is optimised for reads,
and while batches are not fast, they are grouped together
to do less of the expensive peer calculation if there are many
changes rapidly.

Writes will block until commited, while reads are never
blocked.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

integration/auth_oidc_test.go

@@ -176,6 +176,7 @@ func TestOIDCExpireNodesBasedOnTokenExpiry(t *testing.T) {
 			assert.NoError(ct, err)
 			assert.Equal(ct, "NeedsLogin", status.BackendState)
 		}
+		assertTailscaleNodesLogout(t, allClients)
 	}, shortAccessTTL+10*time.Second, 5*time.Second)
 }