feat: add an option to use native timer functions (#672)

This allows to control the behavior of mocked timers (@sinonjs/fake-timers),
depending on the value of the "useNativeTimers" option:

- true: use native setTimeout function
- false (default): use classic timers, that may be mocked

The "installTimerFunctions" method will also be used in the
`socket.io-client` package:

```
import { installTimerFunctions } from "engine.io-client/lib/util";
```

Note: we could also have put the method in its own library, but that
sounded a bit overkill

Related: https://github.com/socketio/socket.io-client/pull/1479