Major Description
Currently during RaftServerProxy#groupRemoveAsync there is no way for stateMachine to know that the RaftGroup will be removed. This Jira aims to add a call in the stateMachine to handle group removal.
It also changes the logic of groupRemoval api to remove the RaftServerImpl from the RaftServerProxy#impls map after the shutdown is complete. This is required to synchronize the removal with the corresponding api of RaftServer#getGroupIds. RaftServer#getGroupIds uses the RaftServerProxy#impls map to get the groupIds.