NH-3956 - native SQL query plan cache fix.

[fredericDelaporte]

Fix and test cases for NH-3956: Native SQL query plan may get wrong plan.

Equals was relying on hashcode equality.

Code taken back prior to that "bad optimization", and compared to hibernate code base (main difference: NHibernate shortcuts on hashcode inequality, not Hibernate).

I have spotted some other discrepancies by the way, but left them untouched: Equals implementations for INativeSQLQueryReturn classes do only test their alias properties on NHibernate side, while Hibernate test all properties.

Test cases only test Equality, they do not showcase query plan cache failures which may result from those Equality failures, contrary to those of NH-3954 for the proxy cache. But there is no reason the query plan cache would not have been affected by those Equals failures.

[fredericDelaporte]

I have re-based only for amending the commit message. It looks like a general failure on Team City side was affecting most tests: the builds triggered by this rebase now looks to be passing. (Already 3 good, while previous builds had them all failed but one.)

[fredericDelaporte]

MySql failure is a timeout. To me, it looks like a test infrastructure frequent failure rather than a regression. The same trouble with the same test happened for PR, though it does not changes the same things at all.

[hazzik]

@fredericDelaporte can you please rebase on top of master?