Fix NH-3150: select id generator improvements

[fredericDelaporte]

Fixes #822
Replaces #110

The select id generator generates the id by getting it from the database with a select query filtered by an unique key. This unique key could only be a single entity property, forcing to use a component if the key has to spawn many properties. The first commit add support for keys spanning many properties without using a component.

The select id generator was not supporting idbags, although some preliminary work was already done for this. The second commit fixes this.

These two commits are based on #110 commit, rebased then split and altered on many points. (Breaking changes removed, redundant casts removed, syntax modernized, multiple properties support added to the key parameter and not only to natural-id, tests adjusted, ...)

[hazzik]

Overall solution looks like a hack (I'm not talking here about the workarounds to avoid breaking changes). It seems there could be a more elegant solution.

[fredericDelaporte]

That is a solution attempting minimal impact, likely because the author has not contributed much to NHibernate. I could have reworked it a bit more.

[fredericDelaporte]

Rebased again. Since it is also about unique key, we could fear some conflicts with changes done for associations with property-ref. But there are none, because here this is about natural-ids, not property-ref.

As proposed on the development group, and since no continued grounded disagreement has been expressed there since one week after raised concerns being addressed, I intend to merge this PR next week (likely on Wednesday).

(I am issuing this notice on PR issued before starting to apply this handling of merges. I will not do it for new PR.)

[hazzik]

The issue noted that this solution is a hack has not been resolved since I posted it.

[fredericDelaporte]

The issue noted that this solution is a hack has not been resolved since I posted it.

In June:

Rebased, and fixup commits addressing review added.

I had reworked it right after your post. But now the fixups are squashed.