Add findTokensByUserName method to JdbcTokenStore

This method adds method which was previously available in [TokenStore](https://github.com/spring-projects/spring-security-oauth/blob/master/spring-security-oauth2/src/main/java/org/springframework/security/oauth2/provider/token/TokenStore.java)
and was changed under gh-182 to:
```java
Collection<OAuth2AccessToken> findTokensByClientIdAndUserName(String clientId, String userName);
```

under this pull request original method was added to [JdbcTokenStore](https://github.com/spring-projects/spring-security-oauth/blob/master/spring-security-oauth2/src/main/java/org/springframework/security/oauth2/provider/token/store/JdbcTokenStore.java) 

See related discussion under [commit comment](https://github.com/spring-projects/spring-security-oauth/commit/c0961fd84bcf895cf00e8e03daf131bd326cd3d7#commitcomment-8410827)