improve escaping characters from multibyte strings

Read the whole character from a multibyte string and then escape it.

while here:
- add more virtual methods for escaping strings to BaseExpression

src/postgresqlexpression.h

@@ -50,11 +50,6 @@ public:
 
 protected:
 
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-private:
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 	void before_field_value_string(const FT & field_type, ModelEnv * model_env) override;
 	void after_field_value_string(const FT & field_type, ModelEnv * model_env) override;