add support for QuerySet.prefetch_related()

[timgraham]

If feasible.

Django test app: prefetch_related

[WaVEV]

The prefect_related injects RawSQL extras using the query's extra method (see here). This feature is tightly coupled with SQL engines, so we likely can't implement it without modifying some base compiler flow. We might consider overriding the get_select method in the base compiler to transform the extra into columns or references instead of a RawQuery but I don't know if it is feasible.

[timgraham]

Thanks for checking. We can defer this for now.