Various fixes.

LINQ fixes for Timestamp operations.
If constant is provided it must be casted to appropriate type or Postgres will not consider it timestamptz.
Minor fix for Hstore parsing (this method should not be used anyway).
Fix for LINQ datasource from multiple property navigation.
Previously only one level was supported. Now collection can be at arbitrary level.
UTC fix for DB interaction. If UTC DateTime is provided, offset can't be used.