[main] Fixed certain cases of bad translation of casts via 'as' operator in LINQ queries

[main] Addressed certain issues of translation connected with comparison with local entity instace within LINQ queries

[main] Fixed rare issues of incorrect translation of filtered index expressions including conditional expressions

[main] Join/LeftJoin is denied to have the same expression instance for both inner/outer selector

[main] Addressed issue when wrong type of join was chosen when .First/FirstOrDefalult() method was used as subquery

[main] Added dedicated exception when RenameFieldHint.TargetType exists in current model but absent in storage model

[main] Xtensive.Sql.Model.NodeCollection<T>.Add() throws understandable exception in case of duplicate name of item

[postgresql] Fixed issue of incorrect translation of contitional expressions including comparison with nullable fields