This fixes an issue with 'partial' suibqueries that they lose `aliasMap` from `q._context`
(QueryBuilder's "internal context") while generating nested `where` sql part.
`QueryBuilder.subqueryOf()` used to grab context from "parent" query, but it doesn't anymore. (since 2019-02-13)
Vincit@524e0be

Basically reintroducing what was deleted in Vincit@524e0be but wrap merging context with a condition that it should only apply to "partial" subqueries + reverse the sequence that `isPartial()` is set before `subqueryOf()`

Co-authored-by: Elan Ruusamäe <[email protected]>

with error: `no such column: model2.model2_prop2`