Optimize GetOrphans and remove wrong checks from IsNotTransientSlow

[bahusoid]

• Removed possibly slow transient check for current items (it's enough to have defined ID for orphan check). Made GetOrphans method not async.

• Made use directly persister.GetIdentifier instead of additional session based checks via GetEntityIdentifierIfNotUnsaved. Removed hack that was introduced with NH607 (when GetEntityIdentifierIfNotUnsaved returns null as ID for not-null transient entity instead of throwing exception)

• Removed wrong checks from IsNotTransientSlow that it seems were added specifically to make old GetOrphans implementation work.

[bahusoid]

PersistenceContext can contain transient objects in state Saving during flush. This hack was added (that returns null ID for transient object) to make GetOrphans work. Removing it as GetOrphans is not relying on GetEntityIdentifierIfNotUnsaved anymore.

[bahusoid]

This call checks session entry dictionary and throw exception for transient object. This exception doesn't make any sense for orphans detection. So let's call persister.GetIdentifier (currently wrapped to ForeignKeys.GetIdentifier to support proxy) directly without any additional checks

[bahusoid]

Transient check is not really required for orphan detection. So remove it as potentially slow

[hazzik]

GetOrphans is still async in PersistentArrayHolder.

[hazzik]

@bahusoid, could you please take a look at this:

GetOrphans is still async in PersistentArrayHolder.

if you have time?

[bahusoid]

@maca88 seems like a bug in AsyncGenerator. I think AsyncGenerator should add Obsolete attribute for abstract method overrides with Obsolete attribute:

nhibernate-core/src/NHibernate/Async/Collection/AbstractPersistentCollection.cs

Lines 131 to 132 in 1006665

	[Obsolete("This method has no more usages and will be removed in a future version")]
	public abstract Task<ICollection> GetOrphansAsync(object snapshot, string entityName, CancellationToken cancellationToken);

But it's not happening leading to warning (that treated as error):

nhibernate-core/src/NHibernate/Async/Collection/PersistentArrayHolder.cs

Line 30 in 1006665

public override Task<ICollection> GetOrphansAsync(object snapshot, string entityName, CancellationToken cancellationToken)

Compilation error:

Async/Collection/PersistentArrayHolder.cs(30,37): error CS0672: Member 'PersistentArrayHolder.GetOrphansAsync(object, string, CancellationToken)' overrides obsolete member 'AbstractPersistentCollection.GetOrphansAsync(object, string, CancellationToken)'. Add the Obsolete attribute to 'PersistentArrayHolder.GetOrphansAsync(object, string, CancellationToken)'. [/home/travis/build/nhibernate/nhibernate-core/src/NHibernate/NHibernate.csproj]

[maca88]

It is a bug, I've opened an issue for it.

[hazzik]

@fredericDelaporte could you please review this PR?