Optimize parser performance

[xingyong]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   financeaccount.zip
   in the attachment，use the go tool pprof profile，will see parser.New() is too hot because of slice allocation.
   
   Maybe, a object pool is proper.

2. What did you expect to see?
   Reduce the overhead of make slice

3. What did you see instead?
   parser.New() is too hot

4. What version of TiDB SQL Parser are you using?
   master, all versions

[tangenta]

The parser instance is expected to be reused.