Provide and use a SimpleMockTimeProvider

Provide a much simpler mock time provider which doesn't offer block_advance et al, and use it in MockRuntime.

And: add some new warning code to MockRuntime, and discourage use of MockSleepProvider.

Rationale

The block_advance functionality is hard to understand and use correctly, and anyway doesn't work with spawned tasks. The MockRuntime with its MockExecutor offers a better (simpler and more faithful) way to ensure that events occur in the right order in tests using mock time.

Additionally, the need to support block_advance makes time.rs complicated and hard to follow. The actual implementation has some problems eg #1036.

Switching MockRuntime to use a simpler fake time provider avoids these problems for new tests (which postdate the availability of MockExecutor).

Future

Ideally I would like to deprecate block_advance et al but they are used fairly extensively in parts of the actual production code (!) via the Runtime trait. I am not inclined to try to rewrite those tests now. But I think new tests should usually use MockRuntime.