Refresh Cache for instances in MessageGenerationPhase while generating messages

[desaikomal]

Describe the bug

A clear and concise description of what the bug is. There are 2 cases where the cache used in MessageGenerationPhase.java can be out of sync with the actual state:

1. Host has died and new host replaces the old host (instanceNames are different)
2. Host has been removed from the cluster

This is a theoretical bug as I don't have any logs but reference to another bug: https://github.com/apache/helix/issues/1802

We don't have logs but the above 2 scenarios needs to be reasoned through code.

To Reproduce

Steps to reproduce the behavior. Please refer to the parent bug: https://github.com/apache/helix/issues/1802

Expected behavior

A clear and concise description of what you expected to happen.

Additional context

Add any other context about the problem here.