Cross-persistence traversal

[romaninsh]

Documentation implies, that when you traverse across persistences, the implied list of IDs will be passed instead of sub-query.

http://agile-data.readthedocs.io/en/develop/references.html#persistence

needs #682

[mvorisek]

Data from a different persistence can be materialized thru php and passed to the selecting/owning Model using "select multiple rows with constants" - https://stackoverflow.com/questions/2201170/how-to-select-multiple-rows-filled-with-constants