ipn/ipnlocal,tka: Fix bugs found by integration testing

* tka.State.staticValidateCheckpoint could call methods on a contained key prior to calling StaticValidate on that key
 * Remove broken backoff / RPC retry logic from tka methods in ipn/ipnlocal, to be fixed at a later time
 * Fix NetworkLockModify() which would attempt to take b.mu twice and deadlock, remove now-unused dependence on netmap
 * Add methods on ipnlocal.LocalBackend to be used in integration tests
 * Use TAILSCALE_USE_WIP_CODE as the feature flag so it can be manipulated in tests

Signed-off-by: Tom DNetto <tom@tailscale.com>

tka/state.go

@@ -249,6 +249,10 @@ func (s *State) staticValidateCheckpoint() error {
 		if err := k.StaticValidate(); err != nil {
 			return fmt.Errorf("key[%d]: %v", i, err)
 		}
+	}
+	// NOTE: The max number of keys is constrained (512), so
+	// O(n^2) is fine.
+	for i, k := range s.Keys {
 		for j, k2 := range s.Keys {
 			if i == j {
 				continue