Standardize adapter metadata passthrough

[TomFrost]

Currently, some adapters pass back the written object on update() or create(), while others pass back a DB response object with metadata (here's looking at you, MySQL).

While the MySQL adapter could be fixed to pass back the written object, the meta information is valuable. I see two clear options for this:

• Symbols. Modli could export const meta = Symbol.for('modli.meta'), and metadata could be accessed by accessing either resolvedObject[modli.meta] or resolvedObject[Symbol.for('modli.meta')]
• For better support with non-ES6, a non-enumerable _meta property could be defined with Object.defineProperty(objToBeResolved, '_meta', {value: metaDataObj}). The only issue there is that it would overwrite the _meta key if it legitimately existed on the target object.

Edit: For clarity, if the second method is chosen, the only change necessary to modli core would be documentation to standardize the convention.