Async helpers called synchronously fail silently

**Sails version**: 1.1
**Node version**: v10.15.1
**NPM version**: 6.4.1
**DB adapter name**: N/A
**DB adapter version**: N/A
**Operating system**: OSX

# Observed

Normal (async) helpers called without `await` keyword fail silently in production environments.

When `process.env.NODE_ENV !== 'PRODUCTION'`, [parley](https://www.npmjs.com/package/parley) prints the following warning to stderr:
```
WARNING: A function that was initially called over 15 seconds
ago has still not actually been executed. Any chance the
source code is missing an "await"?

To assist you in hunting this down, here is a stack trace:
...
```

# Expected

Helper code should run without blocking the caller.

A parallel with normal javascript would be that an `async` function called from synchronous code will still execute. From the node REPL:
```
> (async () => console.log('hi'))()
hi
```

# Recreating

Example project at https://github.com/alxndrsn/sailsjs-helper-silent-fail; see README for usage details.

# Notes

It is actually stated in the [helper docs](https://sailsjs.com/documentation/concepts/helpers#?synchronous-usage) that async helpers called without the `await` keyword _will not run_. Despite this, I still find the behaviour surprising.

Start with the README in the linked reproduction project and the Sails helper documentation, especially the synchronous-usage section. Compare the documented behavior with the production behavior and parley warning, then determine the intended execution semantics and tests needed to show that synchronous calls behave as expected without blocking the caller.

Written by the indexing model from the issue text.