Currently we have two parameters that control how the source of a document
is stored, `enabled` and `synthetic`, both booleans. However, there are only
three possible combinations of these, with `enabled:false` and `synthetic:true`
being disallowed. To make this easier to reason about, this commit replaces
the `enabled` parameter with a new `mode` parameter, which can take the values
`stored`, `synthetic` and `disabled`. The `mode` parameter cannot be set
in combination with `enabled`, and we will subsequently move towards
deprecating `enabled` entirely.