HqlSqlWalker.cs: Remove obsolete handling of collection.elements and collection.indices, syntax which haven't been supported since 2.1.0 (commit 63c5874c6e092fb67468530753e8c5f3eb712e5e).
Hql/Ast/BulkManipulation.cs: Strengthen some tests to avoid testing the corner case that happens to work.
HqlSqlWalker.cs: Remove use of the ASTNode.NextSibling setter, since its name doesn't give clear indication of semantics. And in practice it was the wrong semantics for the current users. See NH-3624.
Merge fix for NH-3624.