Update autoValue to accept async function

[sherryysj]

Is your feature request related to a problem? Please describe.
Meteor have updated their MongoDB functions to async, such as findOneAsync, and if we use these functions in autoValue function, we have to make the autoValue function as async as well. However, it does not work as autoValue seems won't wait for a promise when being calling.

Describe the solution you'd like
autoValue can work for async function

[jankapunkt]

Thank you @sherryysj I checked and this requirement applies to all fields that are computable.

[jankapunkt]

@sherryysj might be of interest to you: #741 (comment)