Improve performance of mysql generator

This change improves performance for generating mysql models
for databases with large number of tables. In my local testing
for a database with about 1000 tables and 140k columns, generation
time was reduced from about 1h to less than one second.

generator/metadata/column_meta_data.go

@@ -6,7 +6,7 @@ import (
 
 // Column struct
 type Column struct {
-	Name         string
+	Name         string `sql:"primary_key"`
 	IsPrimaryKey bool
 	IsNullable   bool
 	IsGenerated  bool

generator/metadata/table_meta_data.go

@@ -2,7 +2,7 @@ package metadata
 
 // Table metadata struct
 type Table struct {
-	Name    string
+	Name    string `sql:"primary_key"`
 	Columns []Column
 }