Synchronize user role retrieval from DB and adding it to user role cache

[DinikaSen]

Purpose

This PR introduces thread synchronization to the process of 'user role retrieval from DB and adding it to user role cache' to avoid multiple threads querying the user roles from DB for the same user concurrently, and update the user role cache with the same value retrieved from DB concurrently.

Resolves : https://github.com/wso2/product-is/issues/10869

[jenkins-is-staging]

PR builder started Link: https://github.com/wso2/product-is/actions/runs/493905882

[jenkins-is-staging]

PR builder completed Link: https://github.com/wso2/product-is/actions/runs/493905882 Status: failure