fix: losing aliasMap in 'partial' subqueries

[falkenhawk]

ported from ovos#13

This fixes an issue with 'partial' subqueries 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) 524e0be

In our case, we were getting SQL errors where alias was necessary to make it work, but it was missing

ER_NON_UNIQ_ERROR: Column 'event_id' in IN/ALL/ANY subquery is ambiguous

Basically reintroducing what was deleted in 524e0be (to fix #1205) but wrap merging context with a condition that it should only apply to "partial" subqueries + reverse the sequence that isPartial() is set before subqueryOf()

[lehni]

@falkenhawk great thanks! this looks reasonable. Adding a test would be good to prevent a future regression?

[falkenhawk]

@lehni I've added a test which fails with

DBError: select `m1`.* from `model2` as `m1` where (`id_col` = (select `m2`.`id_col` from `model2` as `m2` where `m2`.`model2_prop2` = `model2`.`model2_prop2`)) order by `m1`.`model2_prop2` asc - SQLITE_ERROR: no such column: model2.model2_prop2

on the master branch.

Perhaps the ${query.tableRefFor(query.modelClass())}.model2_prop2 reference look weird, but it is useful e.g. for modifiers which add a where condition with subquery, referencing a column from parent query, but the alias which parent query uses is not known and cannot be hardcoded in the modifier.