parser improvement: disallow non-identifier table names

even if it's back-quoted. Porting from Vitess.

Fixes test cases like this:
 select  from t: <nil>, want syntax error at position 9 near '1'
 select  from t: <nil>, want syntax error at position 9 near ':'
 select  from t: <nil>, want syntax error at position 14 near 'table'

Author: Dmitry Voronov

sqlparser/router.go

@@ -5,9 +5,10 @@
 package sqlparser
 
 import (
-	"github.com/maxencoder/mixer/router"
 	"sort"
 	"strconv"
+
+	"github.com/maxencoder/mixer/router"
 )
 
 const (

sqlparser/token.go

@@ -238,7 +238,7 @@ func (tkn *Tokenizer) Scan() (int, []byte) {
 		case '\'', '"':
 			return tkn.scanString(ch, STRING)
 		case '`':
-			return tkn.scanString(ch, ID)
+			return tkn.scanLiteralIdentifier()
 		default:
 			return LEX_ERROR, []byte{byte(ch)}
 		}
@@ -266,6 +266,22 @@ func (tkn *Tokenizer) scanIdentifier() (int, []byte) {
 	return ID, buffer.Bytes()
 }
 
+func (tkn *Tokenizer) scanLiteralIdentifier() (int, []byte) {
+	buffer := bytes.NewBuffer(make([]byte, 0, 8))
+	buffer.WriteByte(byte(tkn.lastChar))
+	if !isLetter(tkn.lastChar) {
+		return LEX_ERROR, buffer.Bytes()
+	}
+	for tkn.next(); isLetter(tkn.lastChar) || isDigit(tkn.lastChar); tkn.next() {
+		buffer.WriteByte(byte(tkn.lastChar))
+	}
+	if tkn.lastChar != '`' {
+		return LEX_ERROR, buffer.Bytes()
+	}
+	tkn.next()
+	return ID, buffer.Bytes()
+}
+
 func (tkn *Tokenizer) scanBindVar() (int, []byte) {
 	buffer := bytes.NewBuffer(make([]byte, 0, 8))
 	buffer.WriteByte(byte(tkn.lastChar))