fix Tokenize concurrent map writes bug

  Test Function:
  var i = 1
  // infinite loop
  for {
	for _, sql := range TestSQLs {
		go ast.QueryType(sql)
	}
	i = i + 1
	fmt.Println("loop: ", i)
  }

ast/token.go

@@ -45,7 +45,6 @@ const (
 var maxCachekeySize = 15
 var cacheHits int
 var cacheMisses int
-var tokenCache map[string]Token
 
 var tokenBoundaries = []string{
 	// multi character
@@ -790,7 +789,7 @@ func Tokenize(sql string) []Token {
 	var token Token
 	var tokenLength int
 	var tokens []Token
-	tokenCache = make(map[string]Token)
+	tokenCache := make(map[string]Token)
 
 	// Used to make sure the string keeps shrinking on each iteration
 	oldStringLen := len(sql) + 1