[5.8] Fix memory leak in JOIN queries

[staudenmeir]

Every JoinClause instance receives and stores the parent query. The instance itself then gets stored in the parent query's $joins property. This circular reference causes a memory leak and can be an issue for long-running scripts.

We can fix it by storing the parent query's connection, grammar, processor and class name separately instead of the whole query instance.

The removed $parentQuery property was private, so this shouldn't be a breaking change.

Fixes #28195.

[laurencei]

Thanks @staudenmeir

[deleugpn]

Maybe it would have been better to have these attributes as private as well to make it possible to fix future bugs without breaking change?

[staudenmeir]

@deleugpn We are only using private properties in very few places (probably not on purpose) and I think we should keep it like that.