Fix locking issues in LUClusterVerifyGroup

- Use functions in ConfigWriter instead of custom loops
- Calculate nodes only once instances locks are acquired, removes one
  potential race condition
- Don't retrieve lists of all node/instance information without locks
- Additionally move the end of the node time check window after the
  first RPC call--the second call isn't involved in checking the
  node time at all

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>