Removes the requirement to use `--trace-events-enabled` to enable
trace events. Tracing is enabled automatically if there are any
enabled categories.

Adds a new `trace_events` module with an API for enabling/disabling
trace events at runtime without a command line flag.

```js
const trace_events = require('trace_events');
const categories = [ 'node.perf', 'node.async_hooks' ];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();
```

Multiple `Tracing` objects may exist and be enabled at any point
in time. The enabled trace event categories is the union of all
enabled `Tracing` objects and the `--trace-event-categories`
flag.