extend @GetDocField to work across databases (@GetDbDocField)

The current @GetDocField function is extremely powerful but is limited to looking up fields only from documents within the same database. It seems to be a natural extension of this to create a new @function to allow cross database field lookups. Although this is available in some small way via @DbLookups, the get doc field functionality is much more targeted and faster than a lookup which cannot be targeted at a specific field on a specific record, but instead returns everything that matches a given key. Performing lookups on a view indexed by unid is a close analog, but seems less efficient than using the existing id tables maintained by the application.