magicsock, ipnlocal: revert eventbus-based node/filter updates, remove Synchronize hack

Restore synchronous method calls from LocalBackend to magicsock.Conn
for node views, filter, and delta mutations. The eventbus delivery
introduced in 8e6f63cf1 was invalid for these updates because
subsequent operations in the same call chain depend on magicsock
already having the current state. The Synchronize/settleEventBus
workaround was fragile and kept requiring more workarounds and
introducing new mystery bugs.

Since eventbus was added, we've since learned more about when to use
eventbus, and this wasn't one of the cases.

We can take another swing at using eventbus for netmap changes in a
future change.

Fixes #16369
Updates #18575 (likely fixes)

Change-Id: I79057cc9259993368bb1e350ff0e073adf6b9a8f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -1600,11 +1600,6 @@ func TestEngineReconfigOnStateChange(t *testing.T) {
 				tt.steps(t, lb, cc)
 			}
 
-			// TODO(bradfitz): this whole event bus settling thing
-			// should be unnecessary once the bogus uses of eventbus
-			// are removed. (https://github.com/tailscale/tailscale/issues/16369)
-			lb.settleEventBus()
-
 			if gotState := lb.State(); gotState != tt.wantState {
 				t.Errorf("State: got %v; want %v", gotState, tt.wantState)
 			}