Potentially memory leak in util/kv_encoder.go

[holys]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   Call initial function many times
   https://github.com/pingcap/tidb/blob/master/util/kvencoder/kv_encoder.go#L211

2. What did you expect to see?
   When initial function exits, release related resources.

3. What did you see instead?
   When initial function exits, it doesn't release related resources.

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Sha1: 6dcaeca

[holys]

cc @winkyao @tiancaiamao

[holys]

#7094