Added flag has_primary_key_set to Model

Now we know whether the primary key is defined or not
and we do not allow to make update/remove if the key is not defined.

And when doing insert/update we can put NULL if child models don't have
the primary key set (fields with has_foreign_key set to true).

Now in after_select() we should also set has_primary_key_set flag
or just call get_last_sequence_for_primary_key instead of get_last_sequence.

fixed: added prefix +00 when serializing PT::Date to PostgreSQL (time zone)
(for a column with a time zone there was a wrong value saved)

src/postgresqlexpression.h

@@ -46,6 +46,8 @@ class PostgreSQLExpression : public DbExpression
 public:
 
 	void esc(char val, PT::TextStream & stream);
+	void esc(const PT::Date & date, PT::TextStream & stream);
+
 	DbExpression & page(PT::TextStream & stream, size_t page_number, size_t page_size);