Allow subselect fetching to be enabled dynamically

[gavinking]

We currently support two APIs (FetchProfile and EntityGraph) which allow association fetching to be overridden (i.e. specified explicitly).

Unfortunately neither of these APIs allow me to ask for an association to be fetched via the SUBSELECT strategy, and that's actually a big missed opportunity. It rarely makes sense to set SUBSELECT as the default fetching strategy, but there are plenty of cases where it would make sense to turn it on for a specific query.

(Arguably, a similar comment applies to batch fetching.)

The thing is that SUBSELECT fetching is actually a pretty cool and unique feature of Hibernate that few know exists, mainly because back years ago I made a dumb decision to have it as a thing you set statically.

If we fix this, it could be way more useful than it is today.

[gavinking]

https://hibernate.atlassian.net/browse/HHH-16651

[gavinking]

So what I've implemented is the following:

• subselect and batch feching may now be controlled globally by config properties
• they may also be controlled by Session methods
• subselect fetching may now be requested by a fetch profile
• and I've allowed fetch profiles to be defined by @FetchProfileOverride annotations on the associations themselves

This is all pretty good now, and I was able to improve a lot of internal code. Especially, LoadQueryInfluencers now really pulls its weight.

I can close this I guess.